Usually not a real problem. Usually there's a stoppage at CCC, and mail restarts in a very short period of time. The truth is that email delivery is usually very very fast, and if delivery isn't happening very very fast, people notice and complain. Waiting a few minutes almost always fixes the trouble.
For mail to function, the department's fileserver (tank1.cs.wpi.edu presently) and csmail.wpi.edu both need to be up and happy. If they are not, that will stop delivery. The network needs to be up also, of course.
Sometimes you will receive a bounce along with the complaint-mail. Look carefully through the text of the bounce as it usually contains many clues as to what happened. Most people that email system@cs asking about mail problems have not read the bounce text at all.
The best approach is to understand the flow of email into and out of the department, and from the report you receive, try to see where the flow is being interrupted.
Send yourself an email! Does it arrive in a reasonable amount of time? If yes, you've established that the trouble is specific to the person reporting the trouble.
If nobody is receiving email at all, throughout the department (can be checked by sending yourself an email and/or checking the logfiles), that can be caused by CCC not sending us email, or by us not listening to CCC.
If many people are receiving email, but one person is not, ask: what problems are attached to this person's setup? Is the user over quota? Is there a disk-full problem somewhere (user home directory, /var/mail)?
Perhaps the user isn't receiving email because the email is being forwarded to some other place via a .forward file in their home directory. Check and see if they have a .forward file.
Sometimes a user isn't complaining to us about CS email at all, and isn't receiving certain items at their CCC account. Class email is normally delivered into a user's CS account, if one exists. So if the user has an account at CS as well as at CCC, the email they're looking for might be here in the department safe-and-sound… but not at CCC where they're looking for it!
With hundreds of users, scanarios like this could go on and on and on. This is why understanding and being able to visualize the email flow is important. There are really only a small number of ways to alter the flow of email on csmail.wpi.edu, the department's email server, once it arrives here.
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An alias entry in /etc/mail/aliases will be used first, if such an alias exists.
A .forward file in a user's home directory will be looked for next, but will only be used if the file is not group or world-writable. If a file is writable by someone other then its owner, sendmail will not trust the .forward file and will act as if the file isn't there. Importantly, .forward files can send to multiple addresses, and can also send a copy while retaining one.
If there is no .forward and no alias, incoming mail will be delivered on csmail into the user's email inbox, /var/mail/[username].