Explicitly decide whether to use TLS in mailer's configuration (#5024)

* explicitly decide on using TLS for mail connections

* explicitly decide on using TLS for mail connections

* keep compatibility

custom/conf/app.ini.sample

 USE_CERTIFICATE = false
 CERT_FILE = custom/mailer/cert.pem
 KEY_FILE = custom/mailer/key.pem
+; Should SMTP connection use TLS
+IS_TLS_ENABLED = false
 ; Mail from address, RFC 5322. This can be just an email address, or the `"Name" <email@example.com>` format
 FROM =
 ; Mailer user name and password

docs/content/doc/advanced/config-cheat-sheet.en-us.md

    HTTP protocol.
 - `USE_COMPAT_SSH_URI`: **false**: Force ssh:// clone url instead of scp-style uri when
    default SSH port is used.
-   
+
 ### Repository - Pull Request (`repository.pull-request`)
 - `WORK_IN_PROGRESS_PREFIXES`: **WIP:,\[WIP\]**: List of prefixes used in Pull Request
  title to mark them as Work In Progress
      `FROM` and `SENDMAIL_PATH`.
 - `SENDMAIL_PATH`: **sendmail**: The location of sendmail on the operating system (can be
    command or full path).
+- ``IS_TLS_ENABLED`` :  **false** : Decide if SMTP connections should use TLS.
 
 ## Cache (`cache`)
 
 - `TOKEN`: **\<empty\>**: You need to specify the token, if you want to include in the authorization the metrics . The same token need to be used in prometheus parameters `bearer_token` or `bearer_token_file`.
 
 ## API (`api`)
- 
-- `ENABLE_SWAGGER_ENDPOINT`: **true**: Enables /api/swagger, /api/v1/swagger etc. endpoints. True or false; default is true. 
+
+- `ENABLE_SWAGGER_ENDPOINT`: **true**: Enables /api/swagger, /api/v1/swagger etc. endpoints. True or false; default is true.
 - `MAX_RESPONSE_ITEMS`: **50**: Max number of items in a page.
 
 ## i18n (`i18n`)

modules/mailer/mailer.go

 	}
 	defer conn.Close()
 
-	isSecureConn := false
+	isSecureConn := opts.IsTLSEnabled || (strings.HasSuffix(port, "465"))
 	// Start TLS directly if the port ends with 465 (SMTPS protocol)
-	if strings.HasSuffix(port, "465") {
+	if isSecureConn {
 		conn = tls.Client(conn, tlsconfig)
-		isSecureConn = true
 	}
 
 	client, err := smtp.NewClient(conn, host)

modules/setting/setting.go

 	SkipVerify        bool
 	UseCertificate    bool
 	CertFile, KeyFile string
+	IsTLSEnabled      bool
 
 	// Sendmail sender
 	UseSendmail  bool
 		UseCertificate: sec.Key("USE_CERTIFICATE").MustBool(),
 		CertFile:       sec.Key("CERT_FILE").String(),
 		KeyFile:        sec.Key("KEY_FILE").String(),
+		IsTLSEnabled:   sec.Key("IS_TLS_ENABLED").MustBool(),
 
 		UseSendmail:  sec.Key("USE_SENDMAIL").MustBool(),
 		SendmailPath: sec.Key("SENDMAIL_PATH").MustString("sendmail"),