When you delete a message in Outlook it marks the message as deleted (putting a line through it). You have to 'purge' the mailbox to actually delete the message.

See this Microsoft article for more information on this.

Note that other IMAP4 email clients such as Mozilla Thunderbird have more options for deleting messages (eg Thunderbird has a choice of: move to 'deleted' folder, delete immediately, or mark as deleted)