How to link between two TN

Hi all,

I face the problem whereby my client need to have a project to communicate with the external Trading Partner, but their network infrastructure not allow TN delivery the email to their SMTP server. This is due with SMTP server is located in Internal Network Zone (TN located in Internet Network Zone).
So, it’s is workable to setup another TN in EAI Zone (EAI Zone able to link with Internet and Internal Zone) as show in below ? When the TN (in Internet Network Zone) need to send email, it will pass throught this EAI Zone’s TN to the SMTP server. Please advise.

TN (Internet Zone) <--> TN (EAI Zone) <--> SMTP Server (Internal Zone).