Aha! That explains why I've had a few dozen calls from clients whose iThings suddenly stopped sending mail over the past two days!
The clients who had problems also were established clients rather than recently-acquired ones, which was a bit of a deviation from the norm. Usually when these mystery mail problems arise with iThings, they're more equal opportunity. But now it makes sense.
The specific problem occurs with devices configured to use Port 465 for secure SMTP using SSL. That hasn't been an officially supported configuration for quite some time. Nonetheless, until recently, many email clients (apparently including Apple's) defaulted to it; and many mail servers (including any that I manage) continued to support it as a legacy configuration (mainly to avoid dealing with the tickets, quite frankly).
The problems I've been fielding for the past two days occur when the device changes the outgoing port to 587, which is now the correct port. Unfortunately, it leaves the encryption protocol configured as SSL rather than changing it to STARTTLS. SSL SMTP over 587 is not a supported configuration and never has been. Ipso facto, the outgoing mail no longer works.
The solution, of course, is to simply change the encryption protocol to STARTTLS to match the port change. That solves the problem. Changing the port back to 465 does NOT solve the problem. The device will change it back again. (Besides, it hasn't been a supported combination in years.)
Long story short, if your iThing suddenly can't send mail after having been able to send it before, make sure that you're configured to use SMTP over Port 587 using STARTTLS. That should solve the problem.
Rich