Why so many mails stuck at Exchange queue?

This is using MS Exchange Servers, 1 Exchange 2K7 and 1 Exchange 2K13 co-exist. This is in the midst of migration, in which all public folders and mailboxes had been moved to 2K13. Recently, the Exch2K7 server has been powered down. Before the decommissioning, I have checked the send and receive connectors, to make sure that the mailflow is in/out the Exch2k13.

However, the users feedback that sometimes, some incoming mails may take a while to receive. No problem with the outgoing mails. After much checking, I can rule out the possibility of spaming, blacklisting, etc. I noticed that in the Queue viewer, sometimes, there are about 60+ mails were stuck there. But, these stuck mails would be disappeared may be in 20 minutes' time. For your info, this mailserver housed 200+ mailboxes, with the only mailbox database of about 150+ gb. This Exch2K13 is a MS HyperV VM, with virtual resources of: 2 vCPU, 16GB vRAM, and 600 GB D:\ for Exchange DB and logs. As for the CPU/RAM usages, are up to 30% CPU, and 9 GB (out of 16 GB) memory usages.

Please see the screenshot of queue as attached. Sometimes, the db01 mailbox database may see up to 60+ queue, this queue may lasts for up to 20 minutes, before getting back to 0.

please help.
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Ganesh Kumar ASr Infrastructure SpecialistCommented:
First you need to properly decommission the exchange 2007 server, i doubt may be  queue is still checking the old exchange server.

Here is the post installation steps to move all the existing role from Exchange 2007 to Exchange 2013 and decommision the legacy server : http://msexchangeguru.com/2013/09/01/e20102007decomposte2013mig/ 
and http://exchangeserverpro.com/exchange-server-2010-2013-migration-removing-legacy-servers/
Will SzymkowskiSenior Solution ArchitectCommented:
I would also agree that Exchange 2007 should be properly removed and not just powerered down. Active Directory will still reference this server even when it is offline/powered off.

What are the emails that are getting stuck in the queue?

MichaelBalackAuthor Commented:
Hi Will,

Those stuck mails are targetting at mailbox database of the Exch2K13 server.
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Budhi WitonoCommented:
Hi Michael, have you installed virtual machine driver? If you use legacy network driver, you should change to network driver. I had similiar problem , i had exchange on the vmware guest and installed network driver vmxnet and it resolved.
MichaelBalackAuthor Commented:
Hi all,

Eventually, it is a DNS resolutions issue. On the hosts file on both DC and Exch2K13, there are few host resolutions which still pointing to the old servers. Removing them all and observed for the past few days, the stuck mail issue seems to be disappeared.

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MichaelBalackAuthor Commented:
By resolving the DNS issue, no more mail stuck problem.
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