When you want to connect your blackberry to the blackberry server, you first have to go through a process called Enterprise Activation. After selecting enterprise activation on your blackberry, an activation e-mail is sent to your inbox. If not, then the e-mail is probably marked as spam and ended up in your junk-box. Solution: whitelist the sender domain.
The blackberry server should then retrieve the activation e-mail from the inbox. If the activation e-mail is not picked-up by the blackberry server while other users are running without problems, then this might be related to errors in the blackberry server PST file. Use SCANPST to verify/fix issues (you might need to run this tool multiple times, until “no errors are found”).
As a last resort, remove the user from the enterprise server, make sure all cache files are deleted for that user, remove the GMAIL Sync application from your device, whipe your blackberry device and start from scratch.
Please note: this applies to blackberry server 5, which connects to GMAIL using the gmail app sync adapter. Maybe this also applies to other system configurations, but I cannot tell for sure.
This SCANPST solution might also sole the problem when at a certain moment the wireles synchronisation does not work anymore.