We upgraded our account from POP to IMAP yesterday. No issues with POP for the past couple years, other than non-sync with other devices. Now that we've "upgraded" to IMAP, Apple Mail locks up about twice an hour. Force Quit is required. Also, with old POP account disabled, old emails don't appear in the Sent folder of the IMAP account. Thanks for any suggestions.
IMAP Accounts are the more advanced email option. If you have a pop account setup somewhere else, there is a chance that account is pulling email to the device that it's setup on. This can cause issues with IMAP due to the fact it's trying to sync to an empty server (pop device just pulled emails). You'll want to make sure you've discontinued any pop email account that you have setup on clients, then re-setup that is IMAP. A lockup might be due to the application being confused why there is no email present to show.