Warren Toomey via TUHS <tuhs(a)tuhs.org> writes: There is a lot that could be and without more information it will be next to impossible to determine what the problem or problems may be. The first question is are you getting bounces, or is the message just getting black holed?? From the TUHS sending server, can you send an email from the command line (for example) outside of the list management software to a known address that could not receive email from the list itself?? If this direct message (just to give it a name) does not make it, then it could indicate that the receiver has blocked the TUHS server (for a ton of possible reasons). If this direct message does make it, but email from the list management software does not, then it is likely that something about how the message is processed (i.e. altered) is triggering the block. A few others have mentioned DMARC, but DKIM may be involved. More or less, if the original sender uses any of the signed email techniques that exist and the TUHS server alters any part of the message it will likely cause the message to fail the signature check. This may include altering the Subject and the From if these bits of the header were covered by the signature. I am by no means an expert here, but am on a number of email lists and this problem comes up from time to time and there has been debate on how or if the list management software should mess with the message. Of course, I would also check to make sure that the message actually made it off of the TUHS server itself. That is, the message isn't sitting in the local queue having failed to contact the destination email server?? Other stranger stuff.. from my logs of incoming TUHS email it looks like you can send via IPv4 and IPv6... is it only one of those sending addresses that fail?? (This may be hard to determine, I realize, but it may be the case that the receiver can't receive one of the addresses families correctly, probably IPv6 if there is a problem). It doesn't look like you are using TLS as a email client sending back out. Is it possible for you to do that?? It might help. I know that some of the stuff I use to keep SPAM out gives a greater credibility to clients that come in via TLS and have a valid certificate where the cert chain validates completely. (I won't debate whether or not this is a good idea or even useful given how simple it is to get Let's Encrypt certs for free, just mostly mentioning that it happens). I am not at all an expert on this topic, but have had my own email domain since 1993 and have seen a whole lot of really strange stuff. -- Brad Spencer - brad(a)anduin.eldar.org - KC8VKS - http://anduin.eldar.org

On Thu, May 04, 2023 at 09:46:51AM +1000, Warren Toomey via TUHS wrote: All, sorry for not responding to your on- and off-list e-mails in the last week. It's been busy. Thank you for all your suggestions. Surprisingly, e-mail to both Hotmail and Outlook are now working again; I've not touched anything in the interim. So I'll tentatively say 'case closed', but I'll look at your suggestions and see if I can improve the MTA and list configuration on the server. Cheers, Warren