Hey Jeff,
I just wanted to follow up on this thread. Technically you can send via the alias from a client using SMTP. But you have to disable SMTP authentication via IP whitelisting.
Heres an example of a script that I made that uses SMTP.
You can whitelist the IP address for SMTP authentication bypass in Settings->Security->Whitelisting
Please let me me know if this helps.
Kind Regards,
Zach Sylvester
Software Developer
SmarterTools Inc.
www.smartertools.com