I understand the security of only allowing existing users to send through the SMTP. It checks the ‘from’ address and if it is not a match for the box’s email address (domain) it is rejected.
I use other SMTP services (such as smtp2go.com) which has no such restriction. I can send through them with any return address so long as the server info in Apple Mail’s ‘outgoing server’ for the account uses the correct username, password.
Is there anyway that can be done on MIAB?