I came across this issue today, a client had the problem where they used public folders in exchange 2010 and when a user actioned an email such as replied or flagged the mail in the public folder then then another user when they went to the same email would not see that the email had been actioned or flagged. Eventually it would update and the two views would sync.I found this issue would only happen on Windows Vista and Windows 7.
I found that it was a problem with the TCP Auto Tuning, apparently this problem occurs if a network hardware device, such as a router, does not support TCP Window Scaling.
OK so to fix this issue simply run the command as seen below.
netsh interface tcp set global autotuninglevel=disabled
There are other options you can set for this flag, see below.
The default auto-tuning level is “normal”, and the possible settings for the above command are:
highlyrestricted: This setting allows the receive window to increase beyond its default value by a small amount only
normal: default value, allows the receive window to grow to accommodate most conditions
experimental: This is for extreme situations only where the TCP value cuold be huge.