I'll try to explain it clearly because it's not easy.

we have a domain test.eu, it's on-prem exchange. We have systems that take the value of the mail attribute and send emails to it. We want to replace the mail attribute @test.eu with @test.elek.eu, but let both mailboxes receive mail from outside and inside.

My solution would be to

Accepted domain + proxyAddresses + mail attribute rewrite

Adding accepted domain
• This way Exchange fully handles the @test.elek.eu ​​address.

Setting ProxyAddresses
The @test.elek.hu address must be added to each user as a secondary alias

• Primary remains: SMTP:user@test.eu
• Secondary: smtp:user@test.elek.eu

Rewriting mail attribute
The mail attribute must be rewritten for each user

• This way the systems see the @test.elek.eu address.
• Mail works on both domains, because proxyAddresses contains both.

Check
• In AD: mail attribute = @test.elek.eu
• In Exchange: proxyAddresses contains both addresses
• Send test mail to both addresses

Set Send connector to send internally received mail to the net and identify where to send it based on the mx record. (because if this is not there, it would not arrive at @test.elek.eu, but at test.eu)

I hope I was able to write my problem well.

Regards,
Johny989