Suppose I have an MDA in example1.com and an MTA in example2.com.Do I need an extra MTA on example1.com?MUA --- MDA  --- MTA --- | Internet | --- MTA --- MDA --- MUA So far I used two MTAs.
Do two MDAs need their own MTAs?
mail transport agent;mail delivery agent
Well, you could host both domains on the same setup, so then it would all be local delivery. The MUA->MDA->MTA->MTA->MDA->MUA chain still applies, since it is all involved with mail routing.