email was spammed, can't remove the huge email.

I have a new customer to order my cpanel hosting at last 2 days.

I found that today his email was spammed by ppl.... all is invalid addres and reach to main email account. now his main email account was store 500MB email. we can't login to his webmail to delete it..
anybody can give me good idea to remove that huge emails? email was spammed, can't remove the huge email.

by the way, how to protect her does not spam by others?

 

 

 

 

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