Mail client hangs while checking email

Knowing what the exact error message is would be the #1 most helpful thing for diagnosing the issue. Often times just googling the error will lead to the solution. That said, which client are you using, and have you tried reproducing the issue with an alternate client?

That is correct. Under the mbox storage format, all email is written to one file, which is why the file must be locked (this prevents multiple processes from writing to the file at once, potentially corrupting the file).

With maildir, emails are written to their own separate files.

Some really good documentation on this can be found here:
http://www.qmail.org/qmail-manual-html/man5/maildir.html

I'd do everything possible to try to reproduce the error message you saw previously, then search google and the cPanel forums. Changes are, someone else has had the exact same issue and the resolution has been posted. If not, post it here and I'm sure it won't be long before someone figures it out.

 

He's right. The best way to start would be to post the error message.

 

what we mean is errors in the exim_mainlog and paniclog files, and in the maillog file. all located in /var/log/