So I have a very interesting scenario....We plan to move to MS Exchange 2013 from a open source email called Firstclass.....we have one domain (we'll call it contoso.com for this example), we had a previous domain setup of contoso-school.org as well......our current email system and acceptable addresses are both of those domain addresses....they are local DNS domains to us.....so for our Exchange we want these addresses to be acceptable in Exchange as well....but we also are going to add a new domain name to Exchange which would be main email address....which would be contososchool.com....however the domain is owned by 3rd party that runs our website with domain name....so was not going to add to our DNS.

I have exchange setup and working as far as being able to send receive external emails and also have our current Firstclass email system able to send email to exchange users....but I cannot get exchange to email the firstclass account....anytime I try, it just immediately shows up under my exchange account (there servers are all apart of the contoso.com domain).....I tried without having the other domain names added in as Acceptable domains but it still does the same thing....I think because the Exchange server is part of the same domain as other email system its causing this...

Has anyone dealt with this type of situation...I hope I explained it ok

Thanks

My first guess would be that you have these addresses defined under your Email Address Policy (Exchange EAC - Mail Flow - Email Address Policy).  If so that means that all users on Exchange are getting stamped with these domains.  So if you look at your mailbox in Exchange, check out your Email Addresses and see if you have something like 'alias@contoso.com'.  If you do, then Exchange sees this as a valid local email address and delivers it to your mailbox.  There are two options here: 

• If you want to keep the domains separate, then modify the email address policy so that Exchange only stamps contososchool.com email addresses and no longer stamps contoso.com on Exchange mailboxes. 
• Uncheck 'Automatically update email addresses based on the email address policy applied to this recipient' and remove the extra email address on each mailbox that needs this functionality. 

Let me know if that works.&