Was bedeutet "Mailplattform"?

The mail platform of a project specifies for which mail service new permissions are created – Exchange or Mailin-Mail. No permissions can be assigned for the other mail service.

If the project's mail platform is changed (e.g. to Exchange), the mail service for existing accounts in the project does not change.

See also the different settings that are required when setting up a mail programme: Was muss für Exchange bei den Mail-Clients an der LMU beachtet werden?