You should be able to do:
Then you can watch login attempts to your account.
tail -f /opt/zimbra/log/audit.log | grep firstname.lastname@example.org
I would guess that there's a mail client somewhere that's constantly trying to login and failing which causes Zimbra to lock your account.
You could also try disabling the failed login lockout for your account from the Admin Console. It's under the advanced tab for your user. I'd definitely turn it back on, but it might be helpful in troubleshooting to know if that's actually the problem.