While Sending an email user can set Organization level pre-defined TAG on that Email, which enables all the recipient of that email to search that email by using that TAG itself.  This capability also automatically clubs all emails together with specific TAG irrespective of sender or subject.   User can also set his personal TAGs also from Settings-Default-Email Tag-Add Tag option.

For example, there are many sales people who receives Purchase order (PO) from customers and then send it to accounts department for billing. Now with TAG feature, all sales team members with their own email address can send a PO to billing team by putting a TAG as PO. Now accounts person or sales team can pull all emails together which are tagged as PO.

These tags will display at user end. At the time of composing email, user can click on advanced setting and can use tag. 

At the receiver end, added tag will display and receiver can also search this mail with same tag.

For E.g if sender has used tag “Banking” at the time of sending email. Then at the receiver end, receiver can see that tag in the bottom of email as well as receiver can search that same email in search section by selecting criteria “Search By Tag”.

If receiver search email by Tag “Banking”, then all the mails related to that tag, will come in resultant list.