Mr google knows everything:
https://support.microsoft.com/en-gb/...ocomplete-list
But the bottom line is that the autocomplete list (once switched on - see above link) is not connected to your address book - it is generated when you *send* emails, so if you've just moved to a new mail client it will probab;ly take a while to "learn" your frequent emailees. There are ways of manually populating the list, but they are not overly user-friendly. The simplest one is to create a test email (with content saying "please ignore this email - I'm just setting up my new email software" and then bcc it to all your favourite emailees (bcc so that they don't all see who else you email to!). After that they should all be in your autocomplete list.
HTH,
PDR