Troubleshooting Thunderbird IMAP - email on server but not in the Inbox

I advised a friend to use the Mozilla Thunderbird mail archive feature to move all his email from his Inbox so that I could move his WordPress web site to a different server. He did that, but to our dismay, only email from 2010 was archived; all email he received this year had disappeared!

I immediately accessed the mail server via Cpanel and Horde webmail program, and saw all the email of 2011 was still on the server, but with a line through each of them. This meant they were in a deleted state. I immediately highlighted all email, and un-deleted them. However, for some reason though I could see all the email using the webmail program, my friend still could not download them in his Inbox! Every time he clicked on Get Mail in Thunderbird, he got the message “No new mail”. But I knew they were there. IMAP is a service that is supposed to keep the Inbox synchronized with mail on the server so that a person can read the same email on any PC with that IMAP email account setup on it. But because of some glitch that did not allow all of the Inbox to be archived properly, IMAP had marked the mail as already downloaded. My friend wanted all the mail back in the Inbox in the previous format as before for reference sake.

After an hour of searching the Internet for an answer and not finding any, I went back to the mail server and thought maybe I could trick IMAP to allow the mail to be downloaded again by highlighting them all and using the copy mail feature in Horde to copy them all on to themselves. I thought this may restore the IMAP attributes which would allow Thunderbird’s Inbox to synchronize again with the old email on the server. It worked! My friend’s Inbox was restored with all the email he received this year.