Is there a way to tell IIS to us another SMTP relay?
Something like this?
Application(s) sends email SMTP relay on server A. Server A sends email to SMTP realy on server B. Server B send mail to recipient.
You can set a gateway to send all remote mails. But i don't think you can set it "by application". So all mails from all applications on server A will be send to server B. As i understand, recipient is not on server B, so Server B need to accept mails from server A that are not for Server B (relaying)
On server B, change relay restriction: Microsoft Technet