Welcome mail send IP-adres instead of domain

A little offtopic Hello,
I’m new in this forum and watch the youtube channel and his way of telling about it and testing two old nas systems let me buy a Synology nas DS920+

With his help of watching the youtube channel I setup my nas :smiley: and now I like to share my space with the family (starting with my siblings) .
So the problem is:

When I create a new user they get a nice welcome email, but it send my internal ip address instead of the domain name.

I can’t find the configuration to change this behavior. Can someone point me in the right direction?

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