The downgrade concern mentioned in the post is moot with server-to-server SMTP because we have to intentionally allow downgrades to account for old mail servers

The downgrade concern mentioned in the post is moot with server-to-server SMTP because we have to intentionally allow downgrades to account for old mail servers. The other concerns with STARTTLS still apply to server-to-server SMTP, but since our only options for server-to-server SMTP are STARTTLS or no encryption at all, we have to just use STARTTLS here.