iPhones Stop synching with Exchange 2007

iPhones are periodically ceasing to synchronize with Exchange Server 2007 (all updates). Phone will operate fine for a period of time, then stop receiving mail. Users get "Cannot Send Mail. An error occurred while delivering this message" when trying to send email. Confirmed that internet service is not dropping.  Later, a barrage of messages will rush in for no apparent reason. Issue MAY be related to the recent renewal of ssl certificate. Tried different phones, different mailboxes, removing/adding mailbox. Problem occurs on Wi-Fi as well as LTE network. Tried removing partnerships in EMC.
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Not a solution but good reading that may lead to a solution:
Thomas GrassiSystems AdministratorCommented:
In EMC or using OWA looking at the mobile devices does it show the last time it synced?

What IOS are you running?

I have seen this occur once in awhile I simply power down the iphone and power it backup then all is good.

Remember the iphone is just like a computer the more things you put on they the more chance for a problem sometime.
cnptechnologiesAuthor Commented:
Thanks for the suggestions. It does show the last time synced. It stops working for a few hours then starts again. rebooting the phone does not resolve the issue. removing/re-adding the account will fix it for a while but problem returns. iPhone 5 & 6 with a variety of IOS's are experiencing the issue. there are some iPhone 4's that appear to NOT be having the problem.

In the article to which Mr. Best linked, it just started working again. bummer.
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cnptechnologiesAuthor Commented:
Try increasing the queue length in the default app pool and MSExchangeSyncAppPool from 1000 to 10000. This was the latest recommendation from MS that cleared up some errors that we were seeing. Users are testing it right now.
cnptechnologiesAuthor Commented:
It's been over a week and this seems to have resolved the issue for us. So to recap:

1. Increase the queue length in the default app
pool and MSExchangeSyncAppPool from 1000 to 10000

2. Check your Exchange virtual directories for stale virtual directories that have a GUID in the
name. Rebuild them if found.

3. We ran iisreset on the server, rebooted the iphones after doing the above. On some devices we removed/re-added the accounts.

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cnptechnologiesAuthor Commented:
We opened a case with Microsoft and worked with them through completion. It was a very difficult to troubleshoot given the lack of standard logging for ActiveSync and the problem being intermittent.
cnptechnologiesAuthor Commented:
Problem has reoccurred. Almost 3 months of no issues and back to square one. Customer is sending time stamps of when the issue occurs along with screenshot for multiple users to help track down errors in logs. Nothing in event, IIS or Exchange logs. It only show when the connection reestablishes. We've increased all counts even higher than before and have queried logs using Log Parser which shows item counts, Resource utilization etc... and no results leading to known issues. Since nothing is showing up in the logs during the time the phones are unable to connect it leaves me to believe either a carrier issue or Firewall. I increased the idle timeout for HTTPS from 30 minutes to 120 to see if that makes a difference and will monitor for now. Have found nothing where carrier has been the cause of such issues.
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