This error is quite self explanatory. It means that Windows reported that a connection attempt or idle connection timed out.
If this error occurs during a connection attempt (eg in a VPOP3 - SMTP Client connection problem error message), then it means that Windows couldn't connect to the ISP mail server within a fixed time (the timeout limit is fixed within Windows itself). This problem can either mean that your ISP is having a problem, that you are connecting through a slow proxy (in this case, try increasing the
Utilities → Misc Settings → Network Tweaks → Connect Attempt Count or
Utilities → Misc Settings → Network Tweaks → Connect Retry Time settings), or that your internet connection is faulty.
If this error occurs whilst a connection is in progress (eg in a VPOP3 - POP3 Client Problem error message), then it means that the connection was idle for a set time - this set time can be changed on the
Utilities → Misc Settings → Network Tweaks page in the VPOP3 settings.