Is there a way to set it up so user@example.com can be a lemmy account and also a mastadon account? I seen people using subdomains like user@lemmy.example.com and user@mastadon.example.com is this nessasary? Could u also set up a matrix account with the same user@example.com? If not what woukd be requured to change to make this possible?
No, not in the example that you’ve set up. The username is always @user@domain. This is like asking if you can have the same email address on Gmail and Hotmail or whatever. You can have the same user part, but not the domain.
People that have used user@x.domain and also user@y.domain is allowed because they’re different domain names.
Technically you sort of can do that with email. Most providers let you verify you own the other email and then use the other provider’s SMTP to send from a different address.
So u cant just do some weird routing rules and make it work.