how do I delete masses of emails at once on my MacBook Pro 0SX 10.7.5?

1. COMMAND-A = Select All
2. Select an item at the top, then hold SHIFT while selecting a second item further down, this will select the items and everything in between them
3. Select an item and then hold COMMAND for each additional item you want to select

then right-click (CTRL-click) and delete

Select them all. CTRL- or RIGHT-click and choose Delete from the context menu. They will be moved into the mail account's trash folder. Select your email Trash folder in Mail's sidebar. Select all messages and Delete them a second time. They will then be gone completely.

Note that because of the large number of messages this may all take a little time to finish up.