4 Dec
2015
4 Dec
'15
3:40 p.m.
The issue is that list posts sent to AOL subscriber addresses are now bouncing as undeliverable with the bounce code: "521 5.2.1 : AOL will not accept delivery of this message."
I see that you have a yahoo.com address. If there's a yahoo.com address on the From: line of the list mail, AOL and Yahoo's well documented abuse of DMARC will cause the failure you're seeing.
If you're running a recent version of Mailman, there are some DMARC workarounds you can use. Other than that, I'd find a different address to mail from, not at Yahoo, not at AOL, and preferably not at Gmail.
R's, John