Given your description of the issue (works on other Macs), it may be worth trying to set up that account by itself in a new test user on the iMac to see if it can be isolated to a user-specific file. You may find the following article helpful:
As far as I can tell so far, and as I expected, the problem does not occur with a new user account.
I also suspected one of the files inside ~/Library/Mail/V2 as the culprit, especially as these do not get deleted if you delete a mail account from within Mail.
However, at one point I deleted the account in Mail _and_ the folder inside ~/Library/Mail/V2 associated with that account, and the problem reappeared after I set up the account again.
I guess this points to a file inside ~/Library/Mail/V2/MailData
Any idea which one could be a likely candidate?