cPanel has offered 2 options for pop/imap mail software for several years. But now Courier is being deprecated and cPanel is now requiring all servers to be upgraded to Dovecot.
You may have received the following notice:
The cPanel & WHM update cannot proceed because the following service has been deprecated: Courier You have 27 day(s) and 23 hour(s) until we remove Courier and replace it with Dovecot. To continue using Courier, you must change your Update Preferences in WHM to Long Term Support (LTS). By switching to LTS, you will not receive new features and eventually will stop receiving security updates. cPanel & WHM version 11.52 will be the last LTS version to support Courier. For more information about Long Term Support: https://go.cpanel.net/longtermsupport
To be able to continue receiving updates and stay current with the latest features of cPanel, it’s best to switch now to Dovecot.
This can easily be done in WHM, go to the link ‘Mailserver Selection’, then on that page click the button next to Dovecot, and click Save at the bottom.
It will automatically switch and convert the entire server. There should be no downtime and no lost mail. Users should not notice any difference and should be completely seamless.