Had trouble for years with yahoo rejecting replys to messages sent from their servers because i did not remove all the yahoo crap.
And had lots of trouble with a couple of ISPs that would reject email sent from our servers. Spent too much time on this stupidity
So, we advised our clients, who used our server, to get REAL email addresses. No more AOL, Comcast, Yahoo,.... We stopped trying to fix what all these idiots were screwing.
(yahaoo got better.)
CW: publish a list of ISPs that are being problematic so we all can get REAL email addresses.
This will eliminate at least one of the newsletter/customer communication problems.