E-mail working on one system but not another

I have two parallel environments set up, and e-mail notifications are working for Joomla and Kunena in both. However, I am only getting notifications from uddeIM in one of the environments.

I am not seeing any configuration differences between the two systems.

I've already confirmed that the users involved are set up to receive e-mails, and that e-mails are turned on for both systems for all users. Both are using phpMail with the same sending address and recipients.

I also added some code to send some output to a file to show progress, and it appears to be getting to the call to actually send the e-mail @mail in iddeUMsendmail with appropriate parameters, so I'm at a loss how to further troubleshoot the problem (as I don't have quite the same level of access on the non-working server).

On the system I'm having issues with, the Sender e-mail is exactly the same as the Joomla From address that is working for other system notifications. There are definitely differences in the mail.log between what uddeIM is sending and what was sent from Kunena (that did arrive):

If there are other relevant bits of info I can provide or something specific I can look at, that would be extremely helpful.

Thanks!

Just to see if it made a difference, I removed the line that sets the reply-to to the recipient's address - and I did start receiving e-mails (even specifying a dummy address that wasn't the recipient's seems to also work). It might be worth adding an optional "reply address" to the configuration.

Strange. Maybe it is a spam filter that filters emails containing a reply-to that is identical to a senders address?

I have a similar problem

PM notifications are being dropped when the notification goes to an outlook.com or hotmail.com address (ie Microsoft). Other emails from my website get through to these addresses so its not the domain or server IP that is an issue. Also pm notifications get through ok to other email addresses eg gmail.com

The email appears to be just dropped by Microsoft - it doesnt even get into the junk folder.

It appears to be an issue wither with the content or the fact the reply to is same as the sender.

I did try including the pm content in the notification but it made no difference so it does point to the issue being around the reply to and sender being the same?