If you’re having trouble adding custom works to the Microsoft Outlook 2016 dictionary, give this solution a try.
- In Outlook, select “File“, then choose “Options“.
- Select “Mail” in the left pane.
- Select “Spelling and Autocorrect…” button.
- Select “Proofing“.
- Select the “Custom Dictionaries…” button.
- Set “CUSTOM.DIC” as the default.
- Ensure the “Dictionary Language” is set to the language you wish to use. Make sure it is the same as the “System Default Language“.
Apply these changes, and adding words to the dictionary should now work fine,.