1.How To Delete All Mail
First, click the tick-box at the top of the page to select all emails on that page.
Look carefully (see image below) and you'll see that a message appears that says 'All XX conversations on this page are selected. Select all XX conversations in XXX'. Click the link in that message to select all messages in Primary(it can be anything else).
Click the rubbish bin/trashcan icon to delete all of them. They'll be moved to Gmail's Bin, so you can always head there to get anything you didn't want to delete back. Be warned, Gmail removes items from the Bin after 30 days. You can use this technique to make a more refined selection and delete specific emails.
2.Hot To Delete Email from a Label
If you've got emails in labels, you can delete all of your emails in a similar way.Select the Label you want from the left-hand panel and then use the tickbox near compose to select all of the messages on one page. If you've got more email, click the 'Select all XXX conversations…' link to select everything. Then hit the Delete button to move everything to the Bin.
3.Delete all email from one contact
If you want to find and delete all email from a contact, just search for that contact. When you've found an email from them, hover the mouse over their name to bring up a pop-up window, then click Email. This will list all emails to and from that person. You can select all of the emails and delete them, in the same way as for Step 3. If you only want to delete email that you sent to that person, or that you received from them, skip to Step 5 to find out how.
4. Delete all unread email
To delete all Unread email, type is:unread or lable:unread into the Search box. This will find any emails that are marked as unread anywhere. Again, you can select and delete all of your emails using the instructions in Step 1. You can use the similar is:read to find all read email, if you only want to leave the messages that haven't got around to reading yet. If you want to only delete unread email from one person or from one label, Step 5 shows you how.
5. Delete all email using a search
As we saw from Step 4, using the Search bar can help you find specific emails. The trick is to know what to type. Here are the most common commands: Label:<name of label> gives you all email from one label, such as label:inbox to search just your inbox; To:<email address>, lets you search for all messages to a person; From:<email address> finds all messages from a person. Don't forget, you can chain multiple requests, so: label:inbox is:unread from:email@example.com' would find all unread email from Bill Gates in your inbox. You can also use 'OR' between search terms. In the example above it would find all email in your inbox, all unread email, all email in your inbox and all emails from Bill Gates, regardless of where they're located. Once you've narrowed your search down, you can select all of the emails that are returned and delete them in one go.
Gmail actually has some very powerful search options built in, above and beyond the ones that I've shown you how to use. Google's excellent Advanced search page shows you how to use all of the different operators to find what you're after. For example, has:attachment, shows you emails with attachments, which can help you clear your inbox of large messages. You can also use the size: filter to find messages bigger than a defined size in bytes: size:1000000 will find any email that is bigger than 1MB; you may better using the larger: or smaller: options, which allow abbreviations. For example, larger:10M will find any message that's larger than 10,000,000 byte (10MB). By using these search options together with my advice, you can quickly pare your inbox down to a more manageable size.