Dual Email Domains on 2013 Exchange - need setup assistance.

First, check your receive connectors.  You should be accepting connections from everyone, unless you use an external spam-filter/antivirus service, in which case you should be accepting connections only from the service. If using such a service, (recommended) it needs to be accepting mail for the domain.

Regarding your MX records, they should be the same as the previous domain. (which still works)  The domain names do not need to be reflected in the name of the server handling the mail, the server just needs to accept and work with the mail.

Hi there,

Thank you..correct the ip is but since its different domain of course the host name is different.
for example :

same public IP of course but
companyA@domain1.com
mx mail.domain1.com

not working
companyB@domain2.com
mx.mail.domain2.com

We do not use a spam filter right now, i checked the default hub transport and its set to allow

thank you sir looks to be working now!

I guess I was trying to understand the logic of why the other way didn't work if I had a local DNS server with a DNS entry for mail.company2.com and on the Name servers had the mx record pointed to the same IP as company1.com

Most likely you were having issues with something like reverse dns lookup not matching.  Many sites now use this as a base authentication to ensure the mail is going to the correct place. A helpful tool for checking is mxtoolbox.com.
If you are going to keep your mail server exposed you may want to look into getting a certificate for it and enable TLS encryption.  I suspect more and more sites will soon be rejecting connections that are not encrypted.  I have several clients that make it part of the contract if you want to do business with them.
Glad it is working.

yes correct we will defiantly use a 3rd party thank you for your help

Don't forget to accept answer and close the thread!

works great!