You can tag words and phrases in Outlook on the web or Outlook.com email messages and calendar events to create a link between items. If you're planning an event, for example, you can add the #conference hashtag to emails about the event and any related meetings. If you select the #conference hashtag in an email or event, you'll be shown all instances of the hashtag—conveniently in one place.
Add hashtags to email messages or meeting requests
On the Subject line or in the body of an email message or calendar event, type a #hashtag symbol and the name of the tag—for example, #seminar—without a space between the symbol and the name.
Tip: When you begin typing with a hashtag, a list appears containing tags that already exist. Either choose a tag from the list, or create a new one.
Type your message.
After you finish creating your email message or calendar event, choose Send.
Search for all items with a hashtag
You don't have to use the Search box to find all instances of a hashtag in your messages or events. Outlook on the web and Outlook.com does the search for you. All you have to do is select a hashtag in an email message or calendar event and a list of all items with that hashtag is displayed.