How to set up Mail so To: auto completes based on Contacts info?
Title is intended to be used within the scope of a company. If you want Mail to recognize names such as “Mom,” try using the Nickname field instead:  



On the macOS version of Contacts, you can go into Contacts → Preferences → Template → Add Field and check Nickname. That field will then become available by default when editing any contact, without requiring you to add it through the + menu.