Why does it take forever to verify SMTP settings on iPhones?

I run a mail server (icewarp 11.2) and iPhones [models 4 and 5, clients using 6 report the same but I haven't gotten my hands on a 6 yet] take forever to verify SMTP settings. Can anyone shed some light on whats going on behind the scenes to make this such a difficult process?

Our mail server runs SSL/TLS encrypted with a valid (not self-signed) cert on port 465. IMAP too is secured and runs on port 993. Incoming mail seems to work great, but when fixing SMTP to use port 465 the phone takes like 5+ minutes. Users say it never actually completes the verification process, but I've found if I hit the home button and send a few mails they eventually squeeze through and things seem to work fine.

What can I provide iThings to make this work better? No other mail clients exhibit this behavior (Thunderbird, Outlook, Android).