Email flow during ex 2010 to ex2016 migration

I'll try to be brief and can provide more info if needed.

I'm doing an on-premise migration from ex2010(sbs server) to ex2016.  I'm following the technet doc that has done a pretty good job for me in the past.  I've completed all the steps and am ready to actually move mailboxes.  

My issue is:
When i point the firewall port 25 to the NEW ex2016, mail is not delivered to mailboxes still on the ex2010 server.  

I've made all of the internal uri's match the external fqdn's;  I've pointed autodiscover to the correct fqdn.  I've changed the connection point and configured outlookanywhere;  All of my internal DNS records for the fqdn are pointing to the new ex2016 server.  I believe the 2 servers are communicating as OWA connects to the old server/interface for mailboxes on the old server.  I acutally migrated the admin mailbox, and OWA give me the interface for the ex2016 server.  All outbound mail works.  With port 25 pointing to the OLD server, all inbound works for mailboxes on old server, a migrated mailbox on new server, and a freshly created mailbox on new server.

I guess i could leave the firewall pointed to the old server and migrate mailboxes as that seems to work well...but it just seems like something is not quite right here and i don't want to compound the issue unneededly.

thoughts appreciated and thanks in advance,
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MASEE Solution Guide - Technical Dept HeadCommented:
Try to send an email to Ex2010 user after pointing your firewall to Ex2016.
What is the error in your Ex2016 queue?
sjonesinAuthor Commented:
I'll give that a try.  I DID telnet to the ex2016 server while the firewall was pointed there and send an email manually...the server responded with the usual "queued for delivery message"...
sjonesinAuthor Commented:
AH...i DO see items stuck in the outbound queue...looks like some type of authentication error...I'll provide details when i have them...thanks
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MASEE Solution Guide - Technical Dept HeadCommented:
Problem is only with outbound emails or both outbound and inbound?

Your email should go out from Ex2016 and email should receive in 2016.

Ensure Exchange server authentication is ticked in the receive connector of both servers.

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sjonesinAuthor Commented:
BAM!  That "exchange server authentication" was the ticket...processed the older emails left from this afternoon when i had the fw pointed to new ex2016.  I'll switch the firewall again tomorrow, but i'm sure that was it.

Ironically, i had scribbled on page 1 of my technet doc "Be Sure 'exchange server' is a valid authentication method".  Unfortunately, it'd been a year since I did one of these and forgot what the hell that meant!

Thank you...I appreciate the expertise.

sjonesinAuthor Commented:
MAS right on point!
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