Use an existing UCS mailstack in Docker Apps

If the App relies on an existing mail infrastructure (IMAP, SMTP), best practice is to set a dependency RequiredAppsInDomain=mailserver in the Apps ini file.

This way the UCS mailstack has to be installed (anywhere in the UCS domain) before the installation of the App is allowed.

Next step is to configure the App to use the UCS IMAP/SMTP server. This should be done in the Apps [[Integration_with_UCS/Join][join script]].