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.