I had several errors reported like this:
SASL LOGIN authentication failed: authentication failure
I searched and googled, and googled and searched... Never found a solution that would solve my problem.
I had a similar problem with imap and pop3 login.
Wanted the user to authenticate as email@example.com instead of user-domain.tld or user.domain.tld
Some research on virtualmin and webmin foruns pointed me to the right direction...
And today, I tried to use the user-domain.tld format and my emails started to be sent.
I've been using qmail since forever, and was trying postfix. Have been testing the spam and virus filtering for some months now, and I decided it was time for a definitive change.
There was the problem with sending emails. I wish someone could point me where to enable the use of: firstname.lastname@example.org in smtp auth.