If any of you use the email function to send messages to other users directly through the forum software, they have not been working right for the last week or so, especially to Yahoo and Gmail users.
I have setup DKIM and DMARC on the server to allow these messages and others to go through to the intended recipient with better chances of getting delivered. One change is that these messages must have a sending address that is allowed by the domain, so all will show the sender as the forum itself (forums@) and not the sending users email address as previously was the case. This is the main reason for failures.
Just make sure that any email you received, if you reply to it, has the reply-to selected correctly by your email client so the messages goes to the original sender and not the forum address.