Solved the problem guys, it wasn't the above. My server hadn't been pertained correctly and apparently the "bar"(?) was full which was causing this problem.
I'm unable to log into my webmail, the page just refreshes back to the log-in screen.
I've not received any emails since 4pm (through apple mail) last night and clients have the same issues, nothing has been changed on the server for around a month, last update to 11.0.9 was on Oct 12...