To delete all, bulk delete, or delete many Gmail messages at once a filter is needed. Create a filter that deletes mail before any date which could include tomorrow if you want to delete everything!
1. Log into Gmail.com
2. Next to the Search the Web button up top, click Create a filter.
3. In the "Has the words" field, type before:2014/01/01. That's just an example date; it would delete all messages received prior to January 1 of last year. You can use any date you want, as long as it conforms to the format YYYY/MM/DD.
4. Click Next step.
5. Check the box marked Delete it, then check the box for Also apply filter to ### conversations below. (Tip: If you want to preserve your mail while still getting it out of your inbox, choose Skip the inbox (Archive it) instead.)
6. Finally, click Create Filter. Depending on how many messages meet the filter criteria, the deletion process may take a minute or two.
Just make sure you really want to permanently delete all that mail. Unless you’re approaching your storage limit, the smarter move is to archive it instead. That way it’ll still be available when your run searches, but won’t clutter up your inbox.
Another way to delete old emails from Gmail
Are you looking to delete/archive/mark as read a whole bunch of emails at once? Good news: you can!
First, decide which emails you want selected and type in a search within your email. That could be all the emails in your inbox (label:inbox). It could be emails from a certain person (from:firstname.lastname@example.org). It could be between certain dates, all sent mail, you decide (check here for info on how to make advanced searches).
Once you’ve made that search in Gmail, go ahead and click the check box in the top left and select All. You’ll see a link pop up just below the button that looks something like this:
Go ahead and click the “select all conversations that match this search” link and take the action you wish to take. Easy as that!