I have Mailstreet as well and use their BES. For approximately the last month, between 10am and 5pm EST I experience delays in processing that range anywhere from 5 minutes to an hour (20 minutes is more typical).
While the delays are ongoing during work hours, one of the symptoms is that my email will come through in groups of 3 or 4.
During the off hours my email delivery is nearly instant.
During regular work hours I can PIN myself and/or send email to my
address instantly.
I've called them probably 4 or 5 times on this issue and all times they've denied a problem.
I have a friend on Mailstreet using a different wireless carrier, and last I checked he was experiencing the same thing.
I would urge you to open a support ticket with Mailstreet (phone seems to be the best way to do this).