If you want to delete many messages at once there are two tips.
1. Sort by 50,100, 300,1000
In the top right corner, you can choose to select 50, 100 or 300 messages at a time. Just click on number, you will get drop down to be able to choose.
2. Select all.
In top left, there is a check box which automatically selects all messages.
(When all messages are selected, you can individual unselect one or two you want to keep.
3. Delete Message
just select messages and then click on delete (either at top or bottom)
- Reducing size of inbox – e.g. removing attachments.
- Using Folders – put saved messages in folder to make it easier to store