To add new mailbox plan, login to the MSPControl and click on Mailbox Plans,
or go to Organization > Exchange > Mailbox Plans.
Click Add New.
Type in the name for a new plan, and select if you want the following to be enabled: POP3, IMAP, OWA/HTTP, MAPI, ActiveSync.
Next, you can decide if you want to hide this plan from addressbook.
Then you can set quotas for this mailbox plan or set them as Unlimited.
Prohibit sending and receiving messages at certain percentage you can in next three fields:
Decide for how long you want deleted items to be stored:
Disable or enable Litigation Hold, set the duration, URL and Hold Message.
Finally, decide on archiving, quota and warning.
When you're done, click Add.
Done! The new mailbox plan has been added and set as default.