Lawrence Finch,
Thank you for your quick response. I understand what you're saying. I was kind of hoping you might be able to provide some insight into what might have happened in my case, given what Apple Support and my email provider said.
Since changing the SMTP Port from 587 to 465 solved my problem (and the expiration problem never occurred on my family member's devices using Port 465), I thought perhaps the email server was (and is) providing the old, expired certificate on Port 587 and the new, renewed certificate on Port 465, but the email provider insists that is not even possible, and they further insist the certificate was never even allowed to expire on the evening of May 14 and that my devices must all have somehow "cached" the old, expired certificate.
If the email provider is giving me the full story, how did my iPhone, iPad, and iMac end up with an incorrect setting (an expired certificate with complete details) if, as Apple Support says, none of these devices cache or save certificates? Why did all three devices fail to obtain the new, renewed certificate? And why is there no mechanism allowing the customer to get rid of a "wrong setting" (a hidden, cached certificate)?
I'm just trying to figure out what I was told, and no amount of searching on the Internet has led me to any explanation at all.