[GH-6366] Add functionality to disable Authentication when connecting to SMTP (#6639)

* Issue #6366: Add feature to disable auth for Encrypted connections to SMTP settings.

* Clean PLAIN AUTH Option

* Reorder SMTP server setup and change helptext

* remove unneeded setting and modify logic

* text description change

webapp/i18n/en.json

@@ -203,8 +203,6 @@
   "admin.compliance_table.userId": "Requested By",
   "admin.connectionSecurityNone": "None",
   "admin.connectionSecurityNoneDescription": "Mattermost will connect over an insecure connection.",
-  "admin.connectionSecurityPlain": "PLAIN",
-  "admin.connectionSecurityPlainDescription": "Mattermost will connect and authenticate over an insecure connection.",
   "admin.connectionSecurityStart": "STARTTLS",
   "admin.connectionSecurityStartDescription": "Takes an existing insecure connection and attempts to upgrade it to a secure connection using TLS.",
   "admin.connectionSecurityTest": "Test Connection",
@@ -309,7 +307,7 @@
   "admin.email.selfPush": "Manually enter Push Notification Service location",
   "admin.email.skipServerCertificateVerification.description": "When true, Mattermost will not verify the email server certificate.",
   "admin.email.skipServerCertificateVerification.title": "Skip Server Certificate Verification: ",
-  "admin.email.smtpPasswordDescription": " Obtain this credential from administrator setting up your email server.",
+  "admin.email.smtpPasswordDescription": "The password associated with the SMTP username.",
   "admin.email.smtpPasswordExample": "E.g.: \"yourpassword\", \"jcuS8PuvcpGhpgHhlcpT1Mx42pnqMxQY\"",
   "admin.email.smtpPasswordTitle": "SMTP Server Password:",
   "admin.email.smtpPortDescription": "Port of SMTP email server.",
@@ -318,9 +316,11 @@
   "admin.email.smtpServerDescription": "Location of SMTP email server.",
   "admin.email.smtpServerExample": "E.g.: \"smtp.yourcompany.com\", \"email-smtp.us-east-1.amazonaws.com\"",
   "admin.email.smtpServerTitle": "SMTP Server:",
-  "admin.email.smtpUsernameDescription": " Obtain this credential from administrator setting up your email server.",
+  "admin.email.smtpUsernameDescription": "The username for authenticating to the SMTP server.",
   "admin.email.smtpUsernameExample": "E.g.: \"admin@yourcompany.com\", \"AKIADTOVBGERKLCBV\"",
   "admin.email.smtpUsernameTitle": "SMTP Server Username:",
+  "admin.email.enableSMTPAuthTitle": "Enable SMTP Authentication:",
+  "admin.email.enableSMTPAuthDesc": "When enabled, username and password are used for authenticating to the SMTP server.",
   "admin.email.testing": "Testing...",
   "admin.false": "false",
   "admin.file.enableFileAttachments": "Allow File Sharing:",