Recommendations for deleting mail
IMAP users will likely find that when a message is deleted it
remains in the mailbox with a line through it or a cross next to it
until deleted messages are purged.
However it is possible to set Thunderbird so that deleted messages are automatically removed.
Tools > Account Settings > Server Settings (located under IMAP account name)
Next to the field 'When I delete a message:' click on the down arrow and select Move it to deleted folder
Also ensure to select Empty Deleted folder on Exit
POP users go to:
Tools > Account Settings > Server Settings (located under POP account name)
Select the option to Empty deleted folder on exit