As you probably know if you’ve stumbled on this posting, you can only have one SMTP server in a ServiceNow instance. I had a requirement where we needed to send mail from two different domains from the same ServiceNow instance, but these domains did not share the same SMTP server. As mail was being sent to a variety of external email services, it needed to send cleanly and without any spoofing or other shenanigans so as not to end up in junk mail folders. So we ended up using Amazon SES.
In Amazon SES, simply define your domains and verify them with DNS entries. You can then send from any address in the domains. The messages generally arrive in the inbox without any authenticity warnings. Note that you have to set up domain identities to get this clean delivery, using email address identities produces warnings about the origin of the messages and they usually end up in junk.
Amazon SES truly is a simple solution that works well.