Try next authentication mechanism if current one failed

[gaol]

In SMTP AUTH Extension specification: https://tools.ietf.org/html/rfc4954#page-6:

If an AUTH command fails, the client MAY proceed without
authentication. Alternatively, the client MAY try another
authentication mechanism or present different credentials by issuing
another AUTH

We can try to switch to next authentication mechanism in the preference allowed lists if current one failed. The first succeeded auth method can act as the preferred one in current MailConfig. If none of those auth methods succeed, it fails the authentication.

Please refer to: https://groups.google.com/forum/#!topic/vertx-dev/pUheXtKOrcc for more information

[gaol]

RFC doc: https://github.com/gaol/vertx-mail-client/wiki/Authentication-mechanism-chain
Work branch: https://github.com/gaol/vertx-mail-client/tree/authentications-chain