How to get iPhone mail to use port 993 (IMAP-over-ssl?)

I'm running a mail server (MailServe Snow) on a Mac Pro (10.6.6; NOT 10.6. Server), and want to allow my clients to get their mail via port 993 and IMAP-over-SSL. By following along with the contents of mail.log and appfirewall.log, I can see this working with my Macs, which are configured to use port 993 when talking to this server. I'm now trying to get iPhones and iPads to do the same thing, and am failing: I specify "Use SSL" and port 993 in the mail account's configuration area, but the phone/pad fails to make its way into the server -- the phone/pad configuration attempt times out, and there's no trace in the logs of the device reaching the server. Connections via regular IMAP (port 143) work fine.

As a possible complication, I've used an entry in /etc/services to move the SSL port away from 22 for security reasons. I have AllowUsers entries in /etc/sshd_config that should let the phones into the server, assuming this is relevant. But, in any case, still no luck.

As is probably obvious, I'm skating pretty close to the edge of my sysadmin competence here; any advice out there would be greatly appreciated. Thanks!