The transition to our new server is nearly complete and it was time to switch over our mailserver. I'd planned ahead and couldn't see too many issues. Everything had been double checked; the email accounts had been duplicated, the email domains moved and finally, the switchover took place.
At this point, every site using a SMTP Authentication module ceased to function and clients started phoning in.
I had previously been through 50 or so of the sites that used the module and updated all details. I'd tested them all and they worked. So, what went wrong?
Clients were stating that they weren't receving emails from their websites, yet when I tried it all worked well. They couldn't all be incorrect.
It turned out that, whilst we'd recreated the email domains and accounts, we hadn't told the DNS where to look for or send the mail so, as it arrived on the server it both existed and didn't at the same time (a bit like Shrodingers fabled cat).
It was only when we forced the server to use either the local server, or a remote server, depending on the clients mailbox destination and MX entries, that the mail came into existence.
Another lesson learning in the black art of server mechanics.....