How to set a custom domain.
You have to set these records in your dns provider. This are in amazon route 53 where I pointed the namecheap custom dns for my domain entusiasmado.com
mail.entusiasmado.com CNAME Simple - No lucy.mxrouting.net 300 - - - mailadmin.entusiasmado.com CNAME Simple - No lucy.mxrouting.net 300 - - - webmail.entusiasmado.com CNAME Simple - No lucy.mxrouting.net 300 - -
you have to set that domain.com mx record points to mx.domain.com .
In amazon you may have to put a number in front of that. 10 mx.domain.com
Go to ssl certificates
Select let´s encrypt
and then unselect the first two options like in the image: