Ok here is what I got...I have two forests artsidd.com and 6teen.com (these are not on the internet just in my lab) 6teen.com also has a child domain called child.6teen.com there is a exchange 2013 setup in this child domain. artsidd.com is the root domain without any child or tree domains under it and this domain also has a separate exchange 2013 setup. I have created the mx records for exchange servers in child domain for this domain's exchange server and also autodiscover A records are in this domain (I tried mx and autodiscover in 6teen.com domain as well but problem didn't go away)
I have setup the send and receive connectors for all the domains on all the exchange servers also all the domains are added to accepted domains.
but when I try to send email from either domain it fails with the message that account doesn't exist.
To recap:
artsidd.com (root domain) mx records and autodiscover records for art-ex1 and 2 are here. stub zones are also present for 6teen.com
art-Ex1 (mailbox and CAS roles)
art-ex2 (mailbox and CAS roles)
6teen.com (empty root domain) stub zone for artsidd.com is also here
child.6teen.com (exchange domain) mx and autodiscover records are here.
chi-ex1 (mailbox and CAS roles)
chi-ex2 (mailbox and CAS roles)
Side note: 6teen.com is short for Windows 2016 server which I'm testing before anyone get some wrong ideas here hahaha any help would be appreciated