I've been running a VPS server for around 3 years. I use postfix to forward email onto my customers. This has worked very well.
Today however I noticed emails are not being forwarded.
I've restarted the server, but that hasn't helped.
An SMTP test on mxtoolbox.com shows everything is working and port 25 is open.
Any ideas what I can look at? I have root SSH access to can run any tests etc...