CiviMail - SMTP error when sending test email
This is a problem not with your SMTP but with your Mail Accounts settings in CiviCRM. When using CiviMail (as opposed to other emails from CiviCRM), the headers are altered so that the message comes from a unique address.  If a message bounces, CiviCRM can know exactly which recipient of which message generated the bounce.

The unique address is generated from the account in Administer - CiviMail - Mail Accounts selecter for bounce processing. It's made up of the Localpart (if set), a "b" followed by random string of numbers and dots, an "@" sign, and the Email Domain.

Looking at the address shown in your error message, it's missing the domain, and that's what the error is complaining about. Check your Mail Accounts settings to be sure you have the domain set.

Incidentally, while you're at it, you should double-check the whole bounce handling system:


Mail to random addresses at the domain you set should end up in a single mailbox.
Your IMAP or POP settings in Mail Accounts should work for checking that mailbox.
Your scheduled job for fetching bounces should be running.


See the page on book.civicrm.org for more information.