If you happen to come across an issue with where an email account’s password no longer works (this can happen with either POP3, IMAP or SMTP) then you can fix this fairly quickly and easily.
Simply login to the cPanel for the domain the email account is added under, select the “Mail” menu, followed by “Manage / Add / Remove Accounts” and then click the “Change Password” link next to the account that isn’t working. You should proceed to reset the password so that it is equal to your old password. Basically, you are just reapplying your old password.
Why does this happen? Sometimes the email address passwords get out of sync with everything else, and to reapply your password resyncs them in the database. This is not a very common problem but it can arise from time to time.