> Hello Taylor. Just as a point of reference, smtp clients that >connect to domains hosted by Microsoft, i.e. outlook.com and any other >domains that use their infrastructure for e-mail, will have to present >a valid SSL certificate in order to submit mail to their smtp servers.
I do not believe this statement is correct. My reading of RFC 8461 is that all it says is that the _server_ has to have a valid certificate and says nothing about client certificates. In my limited experience configuring your SMTP _client_ to present a certificate is very very rare. --Ken