Mailing Bounces 100% with "authentication failure" or "failed to set sender"
I experienced this problem when I was running different versions of PHP in the website and my command line interface (CLI).  You can check your PHP version for the web at Administer menu » Administration Console » System Status.  You can find your CLI PHP version by running php -v.  I was running 7.0 and 7.2 respectively.

Since PHP 7.2 has deprecated the php-mcrypt module, CiviCRM was expecting my mail password to be unencrypted when run from CLI, and encrypted when run from the web interface.  So all web-based testing works - but CLI gets authentication errors.  The solution is to match your PHP version on web and CLI.

Note that if you move from some lower version to PHP 7.2+, you'll need to re-enter your mail password for things to work.