Hello,
Last week, I was setting up ARM for a client and i faced a problem.
For an SMTP server that is out of domain and requires ID/Password authentication, it's not possible to interconnect the ARM server with this SMTP server.
Indeed, the domain field in SMTP server authentication credentials forces search on an AD account that makes it impossible to authenticate out of domain, and it's impossible to let the domain field empty.
The connection with an external server would be to develop.
Thanks