The lookup of the SMTP auth method is done based on symbols, but sometimes the requested value comes from `auth-info', in which case it is a string, so call `intern-soft' to convert it to a symbol (which does nothing if it's already a symbol). * lisp/mail/smtpmail.el (smtpmail-try-auth-methods): Call `intern-soft' on the smtp-auth key's value. (Bug#57373) Do not merge to master