'{storage group name}\Mailbox Store (Server name)': The logical size of this database (the logical size equals the physical size of the .edb file and the .stm file minus the logical free space in each) is {x} GB. This database size has exceeded the size limit of {x} GB.

Cause:
Seen on Exchange 2003 (Including SBS 2003), Note you need to install Exchange 2003 Service Pack 2 before you can proceed. Basically you have exceeded the maximum allowed size of Database that Exchange allows, before SP2 this was limited to 16GB. (Note: That's Exchange 2003 Standard only!! there is NO Limit for Exchange 2003 enterprise). {source}

After SP2 you can increase the limit up to 75GB (Again we are only talking about Exchange 2003 Standard here).

Fix:
1. The First thing that will alert you is mail will stop working, you will see in Exchange System Manager, that the store is dismounted, and you cant mount it. (Trying to, produces the event ID shown above).

2. Start > Run > regedit {enter} Navigate to the following key,

3. Create or Edit the DWORD value "Database Size Limit in Gb" (REMEMBER change the base to Decimal) and set limit accordingly.

4. You should now be able to mount the store.

Comments:

Ref: http://support.microsoft.com/kb/912375/en-gb