Ok, so I did a WebEx with the client myself.. and saw first hand...   

His settings are:  Exchange ActiveSync service.

 

What happens is he loses his inbox.... then it slowly starts to reload.

When it is reloading....  it "loses"  connection for about 10 minutes, then starts again.

He is on Verizon FIOS - 50/50 up/down.

There is no issue of size of hard-drive.

There is probably about 3 years worth of emails (around 10 Gig) on our server.

 

 

How can i trouble shoot on my end......    

In System Administrator go to Settings > Log Settings > Log Detail Levels > Active Sync.. Set that to detailed. Repeat your tests and the logs might give you some more information.

So strange, goes to weird....

[2015.10.20] dd>
[2015.10.20] 12:47:42 [934487991]           <ServerId>7265</ServerId>
[2015.10.20] 12:47:42 [934487991]           <ApplicationData>
[2015.10.20] 12:47:42 [934487991]             <To xmlns="Email">"Lynzy XXX" &lt;lynzyf@XXX-XXXX.com&gt;</To>
[2015.10.20] 12:47:42 [934487991]             <Cc xmlns="Email">johnt@XXXX-starlogistics.com, "Sharon Henson" &lt;s.henson@texasmoving.com&gt;</Cc>
[2015.10.20] 12:47:42 [934487991]             <From xmlns="Email">"Bobby XXXX" &lt;b.XXXX@XXXXXmoving.com&gt;</From>
[2015.10.20] 12:47:42 [934487991]             <Subject xmlns="Email">RE: XXXXX MD to Andover MA 677803</Subject>
[2015.10.20] 12:47:42 [934487991]             <DateReceived xmlns="Email">2014-08-08T21:00:51.922Z</DateReceived>
[2015.10.20] 12:47:42 [934487991]             <DisplayTo xmlns="Email">Lynzy XXXX</DisplayTo>
[2015.10.20] 12:47:42 [934487991]             <ThreadTopic xmlns="Email">RE: GermXXXX MD to Andover MA 677803</ThreadTopic>
[2015.10.20] 12:47:42 [934487991]             <Importance xmlns="Email">1</Importance>
[2015.10.20] 12:47:42 [934487991]             <InternetCPID xmlns="Email">65001</InternetCPID>
[2015.10.20] 12:47:42 [934487991]             <Read xmlns="Email">1</Read>
[2015.10.20] 12:47:42 [934487991]             <LastVerbExecuted xmlns="Email2">0</LastVerbExecuted>
[2015.10.20] 12:47:42 [934487991]             <ContentClass xmlns="Email">urn:content-classes:message</ContentClass>
[2015.10.20] 12:47:42 [934487991]             <MessageClass xmlns="Email">IPM.Note</MessageClass>
[2015.10.20] 12:47:42 [934487991]             <Flag xmlns="Email">
[2015.10.20] 12:47:42 [934487991]               <Status>2</Status>
[2015.10.20] 12:47:42 [934487991]               <FlagType />
[2015.10.20] 12:47:42 [934487991]             </Flag>

 

=============

I am using activesync on my phone as well... and know that I used this morning too... So I find it strange that there were no long entries (even at detail level) .. .until 12.47 (pm).    It was at this time, the user's inbox disappeared... and started all over again.