Intermittent SMTP Auth issues

[cyruspy]

Hi, we're experiencing intermittent authentication issues using Thunderbird as client and sending mail directly from an application using a Zimbra 7.1.1 server.

The thing is, some emails get sent and a few minutes later Thunderbird starts complaining about wrong password.

I've seen a couple of posts with similar issues with Outlook, but no answers..

Extract from logs: Zimbra auth issues - Pastebin.com (initial working auth and later errors)

Any ideas?

[cyruspy]

This is a single server setup, with no more than 100 users...