[TriLUG] [Semi-OT] iOS randomly hates my certificate

[Brian via TriLUG]

Hi Gang,

Tangentially related because my server is Debian!  Seemingly randomly, 
my iPhone (iPhone 7, iOS 15.7.7) complains that it can't verify the 
identity of my mail server, undecidedgames.net.  When I tap the button 
for details and view the certificate, there's nothing wrong with it. 
Not expired, no CN/SAN mismatch, etc., and 99 times in 100, the 
transaction with the server works just fine with no SSL errors.

(Okay, this actual time it was that the certificate expired like, 30 
minutes ago...but it happens more often than that!)

I'm wondering if anyone else in this group has encountered this issue 
and understands a cause or knows a solution.

Cheers!
-Brian