What SMTP header allows me to control the MFROM address used in MTAs?

What SMTP header allows me to control the MFROM address used in MTAs?

I need to send out an email with a custom Reply-To header and has a
particular value for MFROM. (the message envelope)
I have the choice to send though IIS SMTP, or Microsoft Exchange Server,
and both of them relay through a managed service.
IIS -------|
|---- Hosted Provider --- INTERNET ---Salesforce (who
requires a specific MFROM)
Exchange ---|
My goal is to make the MFROM (message envelope) appear as I require (for
Salesforce compatibility).
Question
How can I use Exchange or Microsoft IIS in this configuration to
accomplish this?