After upgrading from OS X Server 10.3 to OS X Server 10.4 the Drupal Mailhandler stopped working on the server. Clients could login and get mail no problem. Even Squirrelmail worked flawlessly.
However when the mailhandler module would try to login the response in the error log was:
TLS server engine: cannot load CA data
TLS server engine: No CA file specified. Client side certs may not work
STARTTLS negotiation failed:
A few sites suggested permission problems with the /etc/certificates directory but that did not seem to be the problem in my case. Ultimately it was fixed by going to Server Admin's general settings and re-creating the server's self-signed certificate.