Outlook is not deleting messages

When using Outlook on an IMAP account, messages are not deleted until you do an extra step of “purging” them. Until then, they may still display in your folder with a strikethrough, and still appear to other devices.

Explanation here: http://email.about.com/od/outlooktips/qt/How_to_Purge_Deleted_Messages_in_Outlook.htm

Setting Outlook to purge deleted messages automatically: https://support.office.com/en-za/article/Purge-IMAP-items-marked-for-deletion-ac1c51da-31c1-435c-a357-96eb78ad684a