Fix: no auth method mailer

[jakerenzella]

WARNING: This change may require servers to set DF_SMTP_AUTHENTICATION to plain as that was the previous default.

Currently, mailer auth defaults to plain.

Some mailers don't rely on mailer auth, and instead use internal IP ranges.

No-auth method smtp relies on the lack of an auth key, and there is no valid value to default to.

The defaul plain method meant that non-auth mailers can't set the auth method correctly.

WARNING: This may require servers to set DF_SMTP_AUTHENTICATION to plain as that was the previous default.

[macite]

Implemented through no_auth / none setting.