I just experienced a similar issue.
I had SMTP Authentication working fine just yesterday, but today it stopped working. In between that I restarted zimbra yesterday evening.
The only log messages I get when trying to send a message and authenticate (the same user can poll IMAP successfully with the same credentials) are :
Code:
May 9 11:02:27 zimbra postfix/smtpd[12033]: warning: SASL authentication failure: Password verification failed
May 9 11:02:27 zimbra postfix/smtpd[12033]: warning: firewall.mycompany.com[192.169.193.236]: SASL PLAIN authentication failed
May 9 11:02:27 zimbra postfix/smtpd[12033]: warning: firewall.mycompany.com[192.169.193.236]: SASL LOGIN authentication failed
I will try restarting the zimbra components and see if that helps.