- Update to the new "$message" API.
- Remove "encoding". I believe "base64" is always the best value for this since we stopped seeing issues once we changed the default.
- Remove "mailer". This is a legacy option that makes little sense given how configuration now works.
- Rename to "SMTP". This doesn't affect users anymore since this mailer has been configured as smtp for about a year.
- This does NOT add a timeout since the SMTP code is inside PHPMailer (see T12404).
Sent messages with many mail features via GMail SMTP and SendGrid SMTP.
The latter -- the base64 is "what format do you want this encoded as in the mail?", not "what format is this data actually encoded as right now".