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Reduce size of Email data safely in Gmail
Following is the practice can be done to reduce usage with deleting old and unwanted emails from Gmail user account.
1. Decide what you want to delete and Create Filter and Label accordingly (by date range or by sender or by size, etc..).
2. Create the Archive of that Label using Google Takeout.
3. Download the Archive and get attached to the “.MBox” reader (thunderbird).
4. Confirm the emails between local Copy and Gmail label to make sure you have secured data.
5. Select the Label in Gmail, select all emails inside that label and delete them and also empty the Trash.
Once you empty the trash there is no such way to restore deleted data back to the user account.
While creating the Filter make sure your search query is specific to exact emails you are looking for.
For your additional help I have created a video tutorial with one example (Emails older than June 1st 2013).
Some useful help articles:
Gmail Advanced search: https://support.google.com/mail/answer/7190
Gmail filters: https://support.google.com/mail/answer/6579?hl=en
Gmail Labels: https://support.google.com/mail/answer/118708?hl=en
Google Takeout FAQ: https://www.google.com/settings/takeout
Deleted messages: https://support.google.com/mail/answer/78353?hl=en