Slow and intermittent mail delivery on BB

[djingo]

Hi all,
I have set up a Bes express 5.0.3 with Zimbra Network edition 6.0.10 on a windows 2008 server. I managed to install, configure everything and activate a blackberry for a user.
The problem is that mails are beeing delivered slowly to the blackberry and sometimes the delivery is intermittent with delays up to 1 hour. I noticed that if i send a mail from the bb device thsi action tipically unlocks the delivery and i receive some mails to.

Can anybody help me out as I already repeated the install on a different zimbra server but had the same problem. ?

Thanks for the help,
Mark