"Domain cannot match hostname" error

Hello together,

my normal mails to something@example.com are managed by a provider.
I want to create a mailcow server that takes care of mails for something@mail.example.com. Mainly for testing purposes. But the hostname for the server should be - for obvious reasons - mail.example.com.
When I install the server and want to add the domain mail.example.com in the admin interface under “E-Mail - Configuration - Add domain”, I get the error message “Domain cannot match hostname”. Any other domain works.
Now I can just accept this and come up with another hostname. But I would like to understand why this is not possible. Is this an internal mailcow “problem”, or something fundamental that doesn’t work with mail?