Normally user@domain1 receives their emails through an SMTP server that is NOT the same “domain1”.
It doesn’t have to be and it usually isn’t.
If it did have to be that way, systems like Gmail, Outlook and Yahoo! would all be overwhelmed by the number of unique entries and corresponding TLS certificates they would require.
So you only really need one FQDN per system.
Just have your users update their MX records to use your service name.
(If you service anyone other than yourself, you are an ESP - act like one - LOL)