Contact pulldown list not showing in TO: field

Hi, My customer has Office 365.  She uses multiple accounts in Outlook.  She has a ton of Contacts, currently stored in a personal Contacts folder called My Contacts.

When she creates a new email message, and types the first letter of the Contact she wants in the TO: field, Outlook should automatically display a pulldown list of all the Contacts that start with that letter.  It displays only the first such Contact.

Any ideas?  Thanks! ... Lou M.