Check your spam filter on your server/network, when the handheld registers to the BESX server the first time Blackberry, from their system email the user an activation email with some encrypted code in it, your BESX server needs to get this email via the users mailbox within a certain amount of time if it doesnt then handset registration fails.
This email normally the user will not see, as the BESX server gets it and deletes it from the users mailbox, its basically a token between the mailbox / BESX server and Blackberry. Sometimes I've seen on slow Blackberry servers that the mail sits in the inbox on the mailbox for a few seconds, and some users who are awake will delete the email on their desktop hence failing the registration.
If you run a spam filter, make sure it allows the mails from blackberry to get to the users mailbox.
This email normally the user will not see, as the BESX server gets it and deletes it from the users mailbox, its basically a token between the mailbox / BESX server and Blackberry. Sometimes I've seen on slow Blackberry servers that the mail sits in the inbox on the mailbox for a few seconds, and some users who are awake will delete the email on their desktop hence failing the registration.
If you run a spam filter, make sure it allows the mails from blackberry to get to the users mailbox.