So currently we have an old IIS SMTP relay used for sending outgoing email from on-prem applications and MFDs that is running into some issues. Primarily, it doesn't support DKIM and it doesn't fall back to sending emails in plain-text when it can't send with TLS.
I believe we're eligible for a free hybrid Exchange license for this purpose through our O365 subscription, but I don't know if it's worth bothering with all the AD schema extensions and whatnot just for using it as a relay server.
