From your Mac you can install a profile for the Apple mail app to use to access your mail.coop account. Here’s how.
Visit https://mail.mail.coop
Log in using your mail.coop username (for example myname@cooptel.net) and password.
On the website page select the ‘Email’ link where it says ‘Apple connection profile’
On the ‘Opening mailcow UI.mobileconfig’ dialog box choose ‘Open with’
Click on OK
Go to System settings from the Apple menu
Choose Privacy & Security on the left and select Profiles on the right
Double-click the downloaded profile
Click on install
Confirm by clicking Continue
Enter your mail.coop password in the dialog box and click Install
The system will ask you for the password again.
Select Continue and complete the password
It’s done.