Contacts you’ve previous addressed envelopes for will be listed on the MRU. If this is the first time using this feature, select Insert Address to browse the Outlook Address Book for an address. You can use Word's Envelopes and Labels feature to print envelopes. If you want to see custom fields in the Word merge field list, make sure you choose All contact fields under Fields to merge in step 3. Save the merge document if you think you might use it again.

The steps are similar in all versions of Outlook, however, the menus are different. You can start the merge from any Outlook contacts folder.