Specify "at once."
Do you mean delete all your mail back through time? Go to Settings > email + accounts and tap on the account name. Change the amount of time that gets fetched with every sync to cut down on how many emails you need to view. Doesn't really delete them from your email account, just from view on your phone. Bulk deletion via browser might be a better option.
Do you mean in real time, as in delete the mail when the notification is dismissed? I don't think that is possible.
Or some other factor I'm not envisioning? While E.B. White would be proud of you following his maxim of 'omit unnecessary words' I feel like myself (and perhaps others) are missing the thrust of the inquiry.