Is there throttling going on here? Constantly queued emails on Hybrid Exch 2013 server with error RecipientThreadLimitExceeded in message tracking logs...

Question

Please let me know if anyone knows an answer to this one... We're in a Hybrid Exchange environment, with 2 x Exchange 2007 servers,  and 1 x Exchange 2013 Hybrid server which is pointing to Office 365 for the purposes of relaying mail to O365 while we
migrate our users out there.
We have just finished migrating, but just a couple of days ago we started experiencing delays in email delivery to O365... Not all mail, but some!  Incoming email or locally generated email gets relayed out through the Hybrid server and out to O365, but
not all email is delayed... only some, but it's constant.  During the busiest part of the day, about 200 messages are sitting in the Queue in Exch2013... but they all eventually resolve between 5 and 45minutes.  The users are not happy.
The last error in the queue viewer for each hung email reads:  451 4.4.0 Temporary server error.  Please try again later.
If I look at the message tracking logs, I find an interesting item -- "RecipientThreadLimitExceeded":
2014-05-15T14:15:51.608Z,192.168.3.11,hydra,207.46.163.215,company-mail-onmicrosoft-com.mail.protection.outlook.com,RecipientThreadLimitExceeded,Outbound
to Office 365,SMTP,DEFER,10307921510617,<[email protected]>,885ea3ce-a020-41b1-8950-08d13e58d6d3,[email protected],451
4.4.0 Temporary server error. Please try again later,10117,1,,,Read: This is your generic subject line,[email protected],[email protected],2014-05-15T14:16:51.608Z,Undefined,,,,S:Microsoft.Exchange.Transport.MailRecipient.RequiredTlsAuthLevel=Opportunistic;S:Microsoft.Exchange.Transport.MailRecipient.EffectiveTlsAuthLevel=EncryptionOnly;S:DeliveryPriority=Normal
I have tried to find some documentation on resolution for this RecipientThreadLimitExceeded error, but I can only come up with some Exchange 2011 documentation which recommends adding some entries to the EdgeTransport.exe.config file to bump up the RecipientThreadLimit
value... I have not found anything pertaining to 2013.  I cannot even find any powershell commands to see what the current RecipientThreadLimit is on 2013!  Aghg!
Has anyone seen this before, or have any recommendations?
Thank you,
Mike

Answer 1

After many days of frustration, Microsoft Support finally resolved this issue.  Believe it or not, but the issue was actually on the Office365 side.  Here's the fix:
Exchange Admin Center -> Mail Flow -> Connectors -> Inbound Connectors
Open your "Inbound from <guid>" with the "On-premises" connector type
Click on Scope -> scroll down to "Associated accepted domains"
We had an entry in there "<organization>.mail.onmicrosoft.com"... Microsoft support had us remove this entry so that the box was completely empty.
That RESOLVED it... amazing what what little entry could do.  We've had this entry in there for about 2 months, and it had been working fine.  Support acknowledged that several customers have had this issue, that they are working on getting it fixed
on the back-end.
Hope this helps somebody... 
-Mike