Add SMTP for sending emails (#1228)

## Tests
Tested for templated mails in user.go, userauth.go, stripe.go,
public_collection.go, playstore.go, admin.go, email_notification.go

Screenshots for all the templates -
[https://drive.google.com/drive/folders/1f7BjM5MHicRUbr5jlCe2-4MGz7s1moLw?usp=sharing](url)

server/configurations/local.yaml

@@ -157,8 +157,24 @@ key:
 jwt:
     secret: i2DecQmfGreG6q1vBj5tCokhlN41gcfS2cjOs9Po-u8=
 
+# SMTP configuration (optional)
+#
+# Configure credentials here for sending mails from museum (e.g. OTP emails).
+#
+# The smtp credentials will be used if the host is specified. Otherwise it will
+# try to use the transmail credentials. Ideally, one of smtp or transmail should
+# be configured for a production instance.
+smtp:
+    host:
+    port:
+    username:
+    password:
+
 # Zoho Zeptomail config (optional)
-# Use case: Sending emails
+#
+# This is an alternative to the `smtp` configuration for sending emails. If this
+# is set (and SMTP credentials are not set), then museum will use the transmail
+# SDK for sending emails using Zoho Zeptomail.
 transmail:
     # Transmail token
     # Mail agent: dev
@@ -279,4 +295,4 @@ jobs:
         # By default, this job is disabled.
         enabled: false
         # If provided, only objects that begin with this prefix are pruned.
-        prefix: ""
+        prefix: ""

server/pkg/utils/email/email.go

@@ -10,6 +10,7 @@ import (
 	"encoding/json"
 	"html/template"
 	"net/http"
+	"net/smtp"
 	"strings"
 
 	"github.com/ente-io/museum/ente"
@@ -20,6 +21,78 @@ import (
 
 // Send sends an email
 func Send(toEmails []string, fromName string, fromEmail string, subject string, htmlBody string, inlineImages []map[string]interface{}) error {
+	smtpHost := viper.GetString("smtp.host")
+	if smtpHost != "" {
+		return sendViaSMTP(toEmails, fromName, fromEmail, subject, htmlBody, inlineImages)
+	} else {
+		return sendViaTransmail(toEmails, fromName, fromEmail, subject, htmlBody, inlineImages)
+	}
+}
+
+func sendViaSMTP(toEmails []string, fromName string, fromEmail string, subject string, htmlBody string, inlineImages []map[string]interface{}) error {
+	if len(toEmails) == 0 {
+		return ente.ErrBadRequest
+	}
+
+	smtpServer := viper.GetString("smtp.host")
+	smtpPort := viper.GetString("smtp.port")
+	smtpUsername := viper.GetString("smtp.username")
+	smtpPassword := viper.GetString("smtp.password")
+
+	var emailMessage string
+
+	// Construct 'emailAddresses' with comma-separated email addresses
+	var emailAddresses string
+	for i, email := range toEmails {
+		if i != 0 {
+			emailAddresses += ","
+		}
+		emailAddresses += email
+	}
+
+	header := "From: " + fromName + " <" + fromEmail + ">\n" +
+		"To: " + emailAddresses + "\n" +
+		"Subject: " + subject + "\n" +
+		"MIME-Version: 1.0\n" +
+		"Content-Type: multipart/related; boundary=boundary\n\n" +
+		"--boundary\n"
+	htmlContent := "Content-Type: text/html; charset=us-ascii\n\n" + htmlBody + "\n"
+
+	emailMessage = header + htmlContent
+
+	if inlineImages == nil {
+		emailMessage += "--boundary--"
+	} else {
+		for _, inlineImage := range inlineImages {
+
+			emailMessage += "--boundary\n"
+			var mimeType = inlineImage["mime_type"].(string)
+			var contentID = inlineImage["cid"].(string)
+			var imgBase64Str = inlineImage["content"].(string)
+
+			var image = "Content-Type: " + mimeType + "\n" +
+				"Content-Transfer-Encoding: base64\n" +
+				"Content-ID: <" + contentID + ">\n" +
+				"Content-Disposition: inline\n\n" + imgBase64Str + "\n"
+
+			emailMessage += image
+		}
+		emailMessage += "--boundary--"
+	}
+
+	// Send the email to each recipient
+	for _, toEmail := range toEmails {
+		auth := smtp.PlainAuth("", smtpUsername, smtpPassword, smtpServer)
+		err := smtp.SendMail(smtpServer+":"+smtpPort, auth, fromEmail, []string{toEmail}, []byte(emailMessage))
+		if err != nil {
+			return stacktrace.Propagate(err, "")
+		}
+	}
+
+	return nil
+}
+
+func sendViaTransmail(toEmails []string, fromName string, fromEmail string, subject string, htmlBody string, inlineImages []map[string]interface{}) error {
 	if len(toEmails) == 0 {
 		return ente.ErrBadRequest
 	}
@@ -69,6 +142,7 @@ func SendTemplatedEmail(to []string, fromName string, fromEmail string, subject
 	if err != nil {
 		return stacktrace.Propagate(err, "")
 	}
+
 	return Send(to, fromName, fromEmail, subject, body, inlineImages)
 }