Vendors With Public Emails

Hello, I have this problem in Multivendor:

If the Vendor has a public email (yahoo, hotmail, gmail...) and the Buyer also has a public email (yahoo, hotmail, gmail...), when cs-cart sends a email to the Buyer "simulating" the email (sender) of the Vendor, the email is returned.

I have tried all options ("via SMTP server", "php mail function"...) but the same problem. I think it is because the public email must be sent from the real account and it can not be sent from another domain changing the sender.

How could I solve this problem?

Thank you.