add SMTP support

it will be used in future update to add email sending capabilities

cmd/root.go

@@ -71,6 +71,7 @@ var (
 )
 
 func init() {
+	rootCmd.CompletionOptions.DisableDefaultCmd = true
 	rootCmd.Flags().BoolP("version", "v", false, "")
 	rootCmd.Version = version.GetAsString()
 	rootCmd.SetVersionTemplate(`{{printf "SFTPGo "}}{{printf "%s" .Version}}

cmd/smtptest.go

@@ -0,0 +1,54 @@
+package cmd
+
+import (
+	"os"
+
+	"github.com/rs/zerolog"
+	"github.com/spf13/cobra"
+
+	"github.com/drakkan/sftpgo/v2/config"
+	"github.com/drakkan/sftpgo/v2/logger"
+	"github.com/drakkan/sftpgo/v2/smtp"
+	"github.com/drakkan/sftpgo/v2/util"
+)
+
+var (
+	smtpTestRecipient string
+	smtpTestCmd       = &cobra.Command{
+		Use:   "smtptest",
+		Short: "Test the SMTP configuration",
+		Long: `SFTPGo will try to send a test email to the specified recipient.
+If the SMTP configuration is correct you should receive this email.`,
+		Run: func(cmd *cobra.Command, args []string) {
+			logger.DisableLogger()
+			logger.EnableConsoleLogger(zerolog.DebugLevel)
+			configDir = util.CleanDirInput(configDir)
+			err := config.LoadConfig(configDir, configFile)
+			if err != nil {
+				logger.WarnToConsole("Unable to initialize data provider, config load error: %v", err)
+				os.Exit(1)
+			}
+			smtpConfig := config.GetSMTPConfig()
+			err = smtpConfig.Initialize()
+			if err != nil {
+				logger.ErrorToConsole("unable to initialize SMTP configuration: %v", err)
+				os.Exit(1)
+			}
+			err = smtp.SendEmail(smtpTestRecipient, "SFTPGo - Testing Email Settings", "It appears your SFTPGo email is setup correctly!",
+				smtp.EmailContentTypeTextPlain)
+			if err != nil {
+				logger.WarnToConsole("Error sending email: %v", err)
+				os.Exit(1)
+			}
+			logger.InfoToConsole("No errors were reported while sending an email. Please check your inbox to make sure.")
+		},
+	}
+)
+
+func init() {
+	addConfigFlags(smtpTestCmd)
+	smtpTestCmd.Flags().StringVar(&smtpTestRecipient, "recipient", "", `email address to send the test e-mail to`)
+	smtpTestCmd.MarkFlagRequired("recipient") //nolint:errcheck
+
+	rootCmd.AddCommand(smtpTestCmd)
+}

cmd/startsubsys.go

@@ -86,6 +86,12 @@ Command-line flags should be specified in the Subsystem declaration.
 				logger.Error(logSender, connectionID, "unable to initialize plugin system: %v", err)
 				os.Exit(1)
 			}
+			smtpConfig := config.GetSMTPConfig()
+			err = smtpConfig.Initialize()
+			if err != nil {
+				logger.Error(logSender, connectionID, "unable to initialize SMTP configuration: %v", err)
+				os.Exit(1)
+			}
 			dataProviderConf := config.GetProviderConf()
 			if dataProviderConf.Driver == dataprovider.SQLiteDataProviderName || dataProviderConf.Driver == dataprovider.BoltDataProviderName {
 				logger.Debug(logSender, connectionID, "data provider %#v not supported in subsystem mode, using %#v provider",

config/config.go

@@ -21,6 +21,7 @@ import (
 	"github.com/drakkan/sftpgo/v2/mfa"
 	"github.com/drakkan/sftpgo/v2/sdk/plugin"
 	"github.com/drakkan/sftpgo/v2/sftpd"
+	"github.com/drakkan/sftpgo/v2/smtp"
 	"github.com/drakkan/sftpgo/v2/telemetry"
 	"github.com/drakkan/sftpgo/v2/util"
 	"github.com/drakkan/sftpgo/v2/version"
@@ -107,6 +108,7 @@ type globalConfig struct {
 	MFAConfig       mfa.Config            `json:"mfa" mapstructure:"mfa"`
 	TelemetryConfig telemetry.Conf        `json:"telemetry" mapstructure:"telemetry"`
 	PluginsConfig   []plugin.Config       `json:"plugins" mapstructure:"plugins"`
+	SMTPConfig      smtp.Config           `json:"smtp" mapstructure:"smtp"`
 }
 
 func init() {
@@ -305,6 +307,16 @@ func Init() {
 			TLSCipherSuites:    nil,
 		},
 		PluginsConfig: nil,
+		SMTPConfig: smtp.Config{
+			Host:       "",
+			Port:       25,
+			From:       "",
+			User:       "",
+			Password:   "",
+			AuthType:   0,
+			Encryption: 0,
+			Domain:     "",
+		},
 	}
 
 	viper.SetEnvPrefix(configEnvPrefix)
@@ -411,6 +423,11 @@ func GetMFAConfig() mfa.Config {
 	return globalConf.MFAConfig
 }
 
+// GetSMTPConfig returns the SMTP configuration
+func GetSMTPConfig() smtp.Config {
+	return globalConf.SMTPConfig
+}
+
 // HasServicesToStart returns true if the config defines at least a service to start.
 // Supported services are SFTP, FTP and WebDAV
 func HasServicesToStart() bool {
@@ -426,19 +443,24 @@ func HasServicesToStart() bool {
 	return false
 }
 
+func getRedactedPassword() string {
+	return "[redacted]"
+}
+
 func getRedactedGlobalConf() globalConfig {
 	conf := globalConf
 	conf.Common.Actions.Hook = util.GetRedactedURL(conf.Common.Actions.Hook)
 	conf.Common.StartupHook = util.GetRedactedURL(conf.Common.StartupHook)
 	conf.Common.PostConnectHook = util.GetRedactedURL(conf.Common.PostConnectHook)
 	conf.SFTPD.KeyboardInteractiveHook = util.GetRedactedURL(conf.SFTPD.KeyboardInteractiveHook)
-	conf.HTTPDConfig.SigningPassphrase = "[redacted]"
-	conf.ProviderConf.Password = "[redacted]"
+	conf.HTTPDConfig.SigningPassphrase = getRedactedPassword()
+	conf.ProviderConf.Password = getRedactedPassword()
 	conf.ProviderConf.Actions.Hook = util.GetRedactedURL(conf.ProviderConf.Actions.Hook)
 	conf.ProviderConf.ExternalAuthHook = util.GetRedactedURL(conf.ProviderConf.ExternalAuthHook)
 	conf.ProviderConf.PreLoginHook = util.GetRedactedURL(conf.ProviderConf.PreLoginHook)
 	conf.ProviderConf.PostLoginHook = util.GetRedactedURL(conf.ProviderConf.PostLoginHook)
 	conf.ProviderConf.CheckPasswordHook = util.GetRedactedURL(conf.ProviderConf.CheckPasswordHook)
+	conf.SMTPConfig.Password = getRedactedPassword()
 	return conf
 }
 
@@ -1146,6 +1168,14 @@ func setViperDefaults() {
 	viper.SetDefault("telemetry.certificate_file", globalConf.TelemetryConfig.CertificateFile)
 	viper.SetDefault("telemetry.certificate_key_file", globalConf.TelemetryConfig.CertificateKeyFile)
 	viper.SetDefault("telemetry.tls_cipher_suites", globalConf.TelemetryConfig.TLSCipherSuites)
+	viper.SetDefault("smtp.host", globalConf.SMTPConfig.Host)
+	viper.SetDefault("smtp.port", globalConf.SMTPConfig.Port)
+	viper.SetDefault("smtp.from", globalConf.SMTPConfig.From)
+	viper.SetDefault("smtp.user", globalConf.SMTPConfig.User)
+	viper.SetDefault("smtp.password", globalConf.SMTPConfig.Password)
+	viper.SetDefault("smtp.auth_type", globalConf.SMTPConfig.AuthType)
+	viper.SetDefault("smtp.encryption", globalConf.SMTPConfig.Encryption)
+	viper.SetDefault("smtp.domain", globalConf.SMTPConfig.Domain)
 }
 
 func lookupBoolFromEnv(envName string) (bool, bool) {

config/config_test.go

@@ -20,6 +20,7 @@ import (
 	"github.com/drakkan/sftpgo/v2/kms"
 	"github.com/drakkan/sftpgo/v2/mfa"
 	"github.com/drakkan/sftpgo/v2/sftpd"
+	"github.com/drakkan/sftpgo/v2/smtp"
 	"github.com/drakkan/sftpgo/v2/util"
 )
 
@@ -42,6 +43,7 @@ func TestLoadConfigTest(t *testing.T) {
 	assert.NotEqual(t, dataprovider.Config{}, config.GetProviderConf())
 	assert.NotEqual(t, sftpd.Configuration{}, config.GetSFTPDConfig())
 	assert.NotEqual(t, httpclient.Config{}, config.GetHTTPConfig())
+	assert.NotEqual(t, smtp.Config{}, config.GetSMTPConfig())
 	confName := tempConfigName + ".json"
 	configFilePath := filepath.Join(configDir, confName)
 	err = config.LoadConfig(configDir, confName)
@@ -340,6 +342,24 @@ func TestSSHCommandsFromEnv(t *testing.T) {
 	}
 }
 
+func TestSMTPFromEnv(t *testing.T) {
+	reset()
+
+	os.Setenv("SFTPGO_SMTP__HOST", "smtp.example.com")
+	os.Setenv("SFTPGO_SMTP__PORT", "587")
+	t.Cleanup(func() {
+		os.Unsetenv("SFTPGO_SMTP__HOST")
+		os.Unsetenv("SFTPGO_SMTP__PORT")
+	})
+
+	configDir := ".."
+	err := config.LoadConfig(configDir, "")
+	assert.NoError(t, err)
+	smtpConfig := config.GetSMTPConfig()
+	assert.Equal(t, "smtp.example.com", smtpConfig.Host)
+	assert.Equal(t, 587, smtpConfig.Port)
+}
+
 func TestMFAFromEnv(t *testing.T) {
 	reset()
 

docs/full-configuration.md

@@ -9,11 +9,14 @@ Usage:
   sftpgo [command]
 
 Available Commands:
-  gen          A collection of useful generators
-  help         Help about any command
-  initprovider Initializes and/or updates the configured data provider
-  portable     Serve a single directory
-  serve        Start the SFTP Server
+  gen            A collection of useful generators
+  help           Help about any command
+  initprovider   Initializes and/or updates the configured data provider
+  portable       Serve a single directory
+  revertprovider Revert the configured data provider to a previous version
+  serve          Start the SFTPGo service
+  smtptest       Test the SMTP configuration
+  startsubsys    Use SFTPGo as SFTP file transfer subsystem
 
 Flags:
   -h, --help      help for sftpgo
@@ -257,6 +260,15 @@ The configuration file contains the following sections:
     - `name`, string. Unique configuration name. This name should not be changed if there are users or admins using the configuration. The name is not exposed to the authentication apps. Default: `Default`.
     - `issuer`, string. Name of the issuing Organization/Company. Default: `SFTPGo`.
     - `algo`, string. Algorithm to use for HMAC. The supported algorithms are: `sha1`, `sha256`, `sha512`. Currently Google Authenticator app on iPhone seems to only support `sha1`, please check the compatibility with your target apps/device before setting a different algorithm. You can also define multiple configurations, for example one that uses `sha256` or `sha512` and another one that uses `sha1` and instruct your users to use the appropriate configuration for their devices/apps. The algorithm should not be changed if there are users or admins using the configuration. Default: `sha1`.
+- **smtp**, SMTP configuration enables SFTPGo email sending capabilities
+  - `host`, string. Location of SMTP email server. Leavy empty to disable email sending capabilities. Default: empty.
+  - `port`, integer. Port of SMTP email server.
+  - `from`, string. From address, for example `SFTPGo <sftpgo@example.com>`. Default: empty
+  - `user`, string. SMTP username. Default: empty
+  - `password`, string. SMTP password. Leaving both username and password empty the SMTP authentication will be disabled. Default: empty
+  - `auth_type`, integer. 0 means `Plain`, 1 means `Login`, 2 means `CRAM-MD5`. Default: `0`.
+  - `encryption`, integer. 0 means no encryption, 1 means `TLS`, 2 means `STARTTLS`. Default: `0`.
+  - `domain`, string. Domain to use for `HELO` command, if empty `localhost` will be used. Default: empty.
 - **plugins**, list of external plugins. Each plugin is configured using a struct with the following fields:
   - `type`, string. Defines the plugin type. Supported types: `notifier`, `kms`, `auth`.
   - `notifier_options`, struct. Defines the options for notifier plugins.

go.mod

@@ -29,7 +29,7 @@ require (
 	github.com/klauspost/compress v1.13.6
 	github.com/kr/text v0.2.0 // indirect
 	github.com/lestrrat-go/backoff/v2 v2.0.8 // indirect
-	github.com/lestrrat-go/jwx v1.2.6
+	github.com/lestrrat-go/jwx v1.2.7
 	github.com/lib/pq v1.10.3
 	github.com/lithammer/shortuuid/v3 v3.0.7
 	github.com/mattn/go-isatty v0.0.14 // indirect
@@ -43,7 +43,7 @@ require (
 	github.com/pkg/sftp v1.13.3
 	github.com/pquerna/otp v1.3.0
 	github.com/prometheus/client_golang v1.11.0
-	github.com/prometheus/common v0.30.0 // indirect
+	github.com/prometheus/common v0.31.0 // indirect
 	github.com/rs/cors v1.8.0
 	github.com/rs/xid v1.3.0
 	github.com/rs/zerolog v1.25.0
@@ -55,6 +55,7 @@ require (
 	github.com/stretchr/testify v1.7.0
 	github.com/studio-b12/gowebdav v0.0.0-20210917133250-a3a86976a1df
 	github.com/wagslane/go-password-validator v0.3.0
+	github.com/xhit/go-simple-mail/v2 v2.10.0
 	github.com/yl2chen/cidranger v1.0.2
 	go.etcd.io/bbolt v1.3.6
 	go.uber.org/automaxprocs v1.4.0

go.sum

@@ -544,12 +544,14 @@ github.com/lestrrat-go/backoff/v2 v2.0.8/go.mod h1:rHP/q/r9aT27n24JQLa7JhSQZCKBB
 github.com/lestrrat-go/blackmagic v1.0.0 h1:XzdxDbuQTz0RZZEmdU7cnQxUtFUzgCSPq8RCz4BxIi4=
 github.com/lestrrat-go/blackmagic v1.0.0/go.mod h1:TNgH//0vYSs8VXDCfkZLgIrVTTXQELZffUV0tz3MtdQ=
 github.com/lestrrat-go/codegen v1.0.1/go.mod h1:JhJw6OQAuPEfVKUCLItpaVLumDGWQznd1VaXrBk9TdM=
+github.com/lestrrat-go/codegen v1.0.2/go.mod h1:JhJw6OQAuPEfVKUCLItpaVLumDGWQznd1VaXrBk9TdM=
 github.com/lestrrat-go/httpcc v1.0.0 h1:FszVC6cKfDvBKcJv646+lkh4GydQg2Z29scgUfkOpYc=
 github.com/lestrrat-go/httpcc v1.0.0/go.mod h1:tGS/u00Vh5N6FHNkExqGGNId8e0Big+++0Gf8MBnAvE=
 github.com/lestrrat-go/iter v1.0.1 h1:q8faalr2dY6o8bV45uwrxq12bRa1ezKrB6oM9FUgN4A=
 github.com/lestrrat-go/iter v1.0.1/go.mod h1:zIdgO1mRKhn8l9vrZJZz9TUMMFbQbLeTsbqPDrJ/OJc=
-github.com/lestrrat-go/jwx v1.2.6 h1:XAgfuHaOB7fDZ/6WhVgl8K89af768dU+3Nx4DlTbLIk=
 github.com/lestrrat-go/jwx v1.2.6/go.mod h1:tJuGuAI3LC71IicTx82Mz1n3w9woAs2bYJZpkjJQ5aU=
+github.com/lestrrat-go/jwx v1.2.7 h1:wO7fEc3PW56wpQBMU5CyRkrk4DVsXxCoJg7oIm5HHE4=
+github.com/lestrrat-go/jwx v1.2.7/go.mod h1:bw24IXWbavc0R2RsOtpXL7RtMyP589yZ1+L7kd09ZGA=
 github.com/lestrrat-go/option v1.0.0 h1:WqAWL8kh8VcSoD6xjSH34/1m8yxluXQbDeKNfvFeEO4=
 github.com/lestrrat-go/option v1.0.0/go.mod h1:5ZHFbivi4xwXxhxY9XHDe2FHo6/Z7WWmtT7T5nBBp3I=
 github.com/lib/pq v1.0.0/go.mod h1:5WUZQaWbwv1U+lTReE5YruASi9Al49XbQIvNi/34Woo=
@@ -679,8 +681,8 @@ github.com/prometheus/client_model v0.2.0/go.mod h1:xMI15A0UPsDsEKsMN9yxemIoYk6T
 github.com/prometheus/common v0.4.1/go.mod h1:TNfzLD0ON7rHzMJeJkieUDPYmFC7Snx/y86RQel1bk4=
 github.com/prometheus/common v0.10.0/go.mod h1:Tlit/dnDKsSWFlCLTWaA1cyBgKHSMdTB80sz/V91rCo=
 github.com/prometheus/common v0.26.0/go.mod h1:M7rCNAaPfAosfx8veZJCuw84e35h3Cfd9VFqTh1DIvc=
-github.com/prometheus/common v0.30.0 h1:JEkYlQnpzrzQFxi6gnukFPdQ+ac82oRhzMcIduJu/Ug=
-github.com/prometheus/common v0.30.0/go.mod h1:vu+V0TpY+O6vW9J44gczi3Ap/oXXR10b+M/gUGO4Hls=
+github.com/prometheus/common v0.31.0 h1:FTJdLTjtrh4dXlCjpzdZJXMnejSTL5F/nVQm5sNwD34=
+github.com/prometheus/common v0.31.0/go.mod h1:vu+V0TpY+O6vW9J44gczi3Ap/oXXR10b+M/gUGO4Hls=
 github.com/prometheus/procfs v0.0.0-20181005140218-185b4288413d/go.mod h1:c3At6R/oaqEKCNdg8wHV1ftS6bRYblBhIjjI8uT2IGk=
 github.com/prometheus/procfs v0.0.2/go.mod h1:TjEm7ze935MbeOT/UhFTIMYKhuLP4wbCsTZCD3I8kEA=
 github.com/prometheus/procfs v0.1.3/go.mod h1:lV6e/gmhEcM9IjHGsFOCxxuZ+z1YqCvr4OA4YeYWdaU=
@@ -762,6 +764,8 @@ github.com/ugorji/go v1.1.7/go.mod h1:kZn38zHttfInRq0xu/PH0az30d+z6vm202qpg1oXVM
 github.com/ugorji/go/codec v1.1.7/go.mod h1:Ax+UKWsSmolVDwsd+7N3ZtXu+yMGCf907BLYF3GoBXY=
 github.com/wagslane/go-password-validator v0.3.0 h1:vfxOPzGHkz5S146HDpavl0cw1DSVP061Ry2PX0/ON6I=
 github.com/wagslane/go-password-validator v0.3.0/go.mod h1:TI1XJ6T5fRdRnHqHt14pvy1tNVnrwe7m3/f1f2fDphQ=
+github.com/xhit/go-simple-mail/v2 v2.10.0 h1:nib6RaJ4qVh5HD9UE9QJqnUZyWp3upv+Z6CFxaMj0V8=
+github.com/xhit/go-simple-mail/v2 v2.10.0/go.mod h1:kA1XbQfCI4JxQ9ccSN6VFyIEkkugOm7YiPkA5hKiQn4=
 github.com/yl2chen/cidranger v1.0.2 h1:lbOWZVCG1tCRX4u24kuM1Tb4nHqWkDxwLdoS+SevawU=
 github.com/yl2chen/cidranger v1.0.2/go.mod h1:9U1yz7WPYDwf0vpNWFaeRh0bjwz5RVgRy/9UEQfHl0g=
 github.com/yuin/goldmark v1.1.25/go.mod h1:3hX8gzYuyVAZsxl0MRgGTJEmQBFcNTphYh9decYSb74=

service/service.go

@@ -110,6 +110,13 @@ func (s *Service) Start() error {
 		logger.ErrorToConsole("unable to initialize plugin system: %v", err)
 		os.Exit(1)
 	}
+	smtpConfig := config.GetSMTPConfig()
+	err = smtpConfig.Initialize()
+	if err != nil {
+		logger.Error(logSender, "", "unable to initialize SMTP configuration: %v", err)
+		logger.ErrorToConsole("unable to initialize SMTP configuration: %v", err)
+		os.Exit(1)
+	}
 
 	providerConf := config.GetProviderConf()
 

sftpgo.json

@@ -254,5 +254,15 @@
       }
     ]
   },
+  "smtp": {
+    "host": "",
+    "port": 25,
+    "from": "",
+    "user": "",
+    "password": "",
+    "auth_type": 0,
+    "encryption": 0,
+    "domain": ""
+  },
   "plugins": []
 }

smtp/smtp.go

@@ -0,0 +1,145 @@
+// Package smtp provides supports for sending emails
+package smtp
+
+import (
+	"errors"
+	"fmt"
+	"time"
+
+	mail "github.com/xhit/go-simple-mail/v2"
+
+	"github.com/drakkan/sftpgo/v2/logger"
+)
+
+const (
+	logSender = "smtp"
+)
+
+// EmailContentType defines the support content types for email body
+type EmailContentType int
+
+// Supporte email body content type
+const (
+	EmailContentTypeTextPlain EmailContentType = iota
+	EmailContentTypeTextHTML
+)
+
+var (
+	smtpServer *mail.SMTPServer
+	from       string
+)
+
+// Config defines the SMTP configuration to use to send emails
+type Config struct {
+	// Location of SMTP email server. Leavy empty to disable email sending capabilities
+	Host string `json:"host" mapstructure:"host"`
+	// Port of SMTP email server
+	Port int `json:"port" mapstructure:"port"`
+	// From address, for example "SFTPGo <sftpgo@example.com>"
+	From string `json:"from" mapstructure:"from"`
+	// SMTP username
+	User string `json:"user" mapstructure:"user"`
+	// SMTP password. Leaving both username and password empty the SMTP authentication
+	// will be disabled
+	Password string `json:"password" mapstructure:"password"`
+	// 0 Plain
+	// 1 Login
+	// 2 CRAM-MD5
+	AuthType int `json:"auth_type" mapstructure:"auth_type"`
+	// 0 no encryption
+	// 1 TLS
+	// 2 start TLS
+	Encryption int `json:"encryption" mapstructure:"encryption"`
+	// Domain to use for HELO command, if empty localhost will be used
+	Domain string `json:"domain" mapstructure:"domain"`
+}
+
+// Initialize initialized and validates the SMTP configuration
+func (c *Config) Initialize() error {
+	smtpServer = nil
+	if c.Host == "" {
+		logger.Debug(logSender, "", "configuration disabled, email capabilities will not be available")
+		return nil
+	}
+	if c.Port <= 0 || c.Port > 65535 {
+		return fmt.Errorf("smtp: invalid port %v", c.Port)
+	}
+	if c.AuthType < 0 || c.AuthType > 2 {
+		return fmt.Errorf("smtp: invalid auth type %v", c.AuthType)
+	}
+	if c.Encryption < 0 || c.Encryption > 2 {
+		return fmt.Errorf("smtp: invalid encryption %v", c.Encryption)
+	}
+	from = c.From
+	smtpServer = mail.NewSMTPClient()
+	smtpServer.Host = c.Host
+	smtpServer.Port = c.Port
+	smtpServer.Username = c.User
+	smtpServer.Password = c.Password
+	smtpServer.Authentication = c.getAuthType()
+	smtpServer.Encryption = c.getEncryption()
+	smtpServer.KeepAlive = false
+	smtpServer.ConnectTimeout = 10 * time.Second
+	smtpServer.SendTimeout = 30 * time.Second
+	if c.Domain != "" {
+		smtpServer.Helo = c.Domain
+	}
+	logger.Debug(logSender, "", "configuration successfully initialized, host: %#v, port: %v, username: %#v, auth: %v, encryption: %v, helo: %#v",
+		smtpServer.Host, smtpServer.Port, smtpServer.Username, smtpServer.Authentication, smtpServer.Encryption, smtpServer.Helo)
+	return nil
+}
+
+func (c *Config) getEncryption() mail.Encryption {
+	switch c.Encryption {
+	case 1:
+		return mail.EncryptionSSLTLS
+	case 2:
+		return mail.EncryptionSTARTTLS
+	default:
+		return mail.EncryptionNone
+	}
+}
+
+func (c *Config) getAuthType() mail.AuthType {
+	if c.User == "" && c.Password == "" {
+		return mail.AuthNone
+	}
+	switch c.AuthType {
+	case 1:
+		return mail.AuthLogin
+	case 2:
+		return mail.AuthCRAMMD5
+	default:
+		return mail.AuthPlain
+	}
+}
+
+// SendEmail tries to send an email using the specified parameters.
+// If the contentType is 0 text/plain is assumed, otherwise text/html
+func SendEmail(to, subject, body string, contentType EmailContentType) error {
+	if smtpServer == nil {
+		return errors.New("smtp: not configured")
+	}
+	smtpClient, err := smtpServer.Connect()
+	if err != nil {
+		return fmt.Errorf("smtp: unable to connect: %w", err)
+	}
+
+	email := mail.NewMSG()
+	if from != "" {
+		email.SetFrom(from)
+	}
+	email.AddTo(to).SetSubject(subject)
+	switch contentType {
+	case EmailContentTypeTextPlain:
+		email.SetBody(mail.TextPlain, body)
+	case EmailContentTypeTextHTML:
+		email.SetBody(mail.TextHTML, body)
+	default:
+		return fmt.Errorf("smtp: unsupported body content type %v", contentType)
+	}
+	if email.Error != nil {
+		return fmt.Errorf("smtp: email error: %w", email.Error)
+	}
+	return email.Send(smtpClient)
+}