1000 points Exchange 5.5 Admin account

I just got into the admin mailbox cause it hasn’t been done in awhile and saw that there are 430,000 messages in there and tried to highlihgt all and delete but couldn’t. I have to delete groups of like 3,000 and that is going to take too long.
How can I delete these emails in the easyest way.
I think I can’t delete the main admin account in exchange but I will try that first while I am waiting for an answer.
Points will be double for the person that answers in the next day with the right answer.