For clarity, I was never suggesting there was an issue with the SME product. I was merely indicating that seb's experience was mirrored by my own. When I have issues I often turn to this forum to see if they've been reported by anyone else; it's an important diagnostic step, IMO, so I feel there's value in somebody saying "Yeah, me too!".
But, to reiterate, this is definitely a change in iOS 9. I was able to connect to my 7.6 box without issue from devices still running iOS 8. We don't officially support iOS so I'm not obligated to try to fix this, but nonetheless it's a pain in the a$$ and I'd prefer that there be an option to patch the SSL issue rather than have to upgrade the entire machine. Is there nothing from CentOS on this?
IIRC, jumping to SME 8 will force all internal clients to authenticate for SMTP, so that will also mean touching all inside clients to make that change, too.