Postfix Help

[david]

Folks

I recently switched from Centos/Sendmail to Ubuntu 
20.04.2/Postfix.  Most of the functions I need are working except 
reading mail from an iPhone.  The mail server is on my own machine, 
and the mail client is on my iPhone.  I use Secure POP.
The symptom I think I see on the server is an authentication failure, 
but I'm sure my passwords are correct (I do control both the iPhone 
and the server).  What information would help diagnose and hopefully 
fix the problem?
Thanks for your assistance.

David