Exchange Backup Strategy…….?

Dear All,

Right now I am using Exmerge to take backup of all mailboxes in existing exchange server 2003. Circular logging is enabled on default First Storage Group.
Now the problem is some users are exceeding the mailbox size of 2 GB, thus Exmerge denied / failed to backup that mailboxes exceeded to 2 GB.
Can any one please let me know what all backup strategy should I opt to take backup of exchange database.
I have enabled Volume Shadow Copy Services on C drive and the Exchsrvr is on D: drive.
Which backup software (Free & Paid) can be used?
Hoping to receive favorable answer.

Thanks!