There could be two things going on. The first is that the Active Sync isn't setup to go beyond a certain amount of days or size. The other is that the e-mails are too big to fit on the Blackjack; in which case you would have to modify the registry on the WM device to move the e-mails and attachments to the memory card.
I have this done on mine, where all e-mails and attachments are saved to my memory card by default, which is great because I now have a 2gb limit on my storage.
Check this

1) Open Active Sync with the (a) phone connected.
2) Click on Tools / Options
3) Click on E-Mail, then Settings
4) Adjust the settings to what you want to sync.


Modifying the default storage for e-mails
This is for advanced users only! I am not responsible if you screw up the registry and have to reload the ROM for your phone!

Instructions on how to move the email email and attachments to your storage card instead of main memory:

1. Create a 2 folders on your storage card.
\Storage Card\Mail\
\Storage Card\Mail\Attachments\

2. Modify HKEY_Current_User\Software\Microsoft\MAPI. Create 2 new string values. The first should be called PropertyPath and should have a value matching the mail folder you created. The second should be called AttachPath and should have a value matchings the attachments folder you created. (ie. \Storage Card\Mail and \Storage Card\Mail\Attachments)

You'll need to restart your phone and resync. Make sure to delete the files off the old locations in main memory as they don't get deleted automatically.
Also, don't remove the memory card while the phone is on after you make this modification. I will mess up, and you will have to change this again!
To go back to default (storing on the phone), just delete the keys that were just made and reboot

I have several Blackjack I's & II's. I am using Activesync with Exchange to sync their emails. It appears to be syncing well with 1 problem. Users cannot see all of their old emails in their inbox. It seems like there is a qty or size limitation on the sync, but I cannot find where to change this. Has anyone else come across this? Any ideas? Thanks, in advance.