This article suggests that there's no point in having a valid SMTP cert. However, consider end-users' clients, which store the SMTP domain (i.e. don't do MX loo

This article suggests that there's no point in having a valid SMTP cert. However, consider end-users' clients, which store the SMTP domain (i.e. don't do MX lookups) and connect to it directly. For mail to users on the same email network, this is the only non-local SMTP hop. Securing this connection also prevents anyone on the end-user's local network from MitMing.