Since the recent changes, PHP’s “mail()” function no longer works. The function returns 1 (i.e., success), but the specified email does not get sent.
This was a sneaky/challenging problem. This wasn’t actually a php problem at all. Long story short: the campus mail relay is IPv4-only and the new vhost backends are IPv6-only. We had at least two distinct problems with our tayga/NAT64 that allows such a setup to work. While I believe it now works, it would be nice to (a) have confirmation, and (b) have robust monitoring so we know if our tayga setup falls over again. We will add (b) to our work queue.
Mail from PHP now works once again; thanks.