Outlook suggests a stranger you emailed once in 2021, but not the client whose contact you saved yesterday. That is not a bug. AutoComplete and Contacts are two separate systems that never promised to match - and understanding the difference ends most autofill confusion in one read.
Ask most Outlook users where the To-field suggestions come from and they say my contacts. They do not. The suggestions come from a small private memory file that Outlook keeps for itself, built only from addresses you have actually used. This page separates the two systems honestly, because half of all autofill problems are really this one misunderstanding.
Every time you send an email in classic Outlook, the address you used is quietly noted in a private file on your PC - the Stream_Autocomplete cache. Outlook does not ask your permission and does not create a contact. It is simply a memory of addresses that have passed through your To, Cc and Bcc fields. Next time you type the first letters of that name, the memory is what pops up. It even remembers addresses you typed once, years ago, and addresses that turned out to be wrong.
Contacts are deliberate. You (or a phone sync, or your office directory) saved a person with a name, address, phone number and notes. Contacts live in a contacts folder inside your mailbox or PST file, they sync to your phone, and they survive moving to a new PC when the account syncs. Outlook Search People and the Address Book read from contacts. AutoComplete does not - which is why a contact you saved carefully may still not be suggested until you have emailed them once.
Treat AutoComplete as a convenience, never as storage. It caps out at roughly 1,000 entries, it silently drops the oldest names when it fills, and it can be wiped by one wrong button press. Anyone whose address truly matters belongs in your contacts, where it is backed up with the mailbox. A good habit: when AutoComplete suggests someone you email weekly and they are not a contact yet, right-click the suggestion's person in a received mail and choose Add to Outlook Contacts. Two different tools, two different jobs - once you see that, autofill behaviour stops looking random.
The video shows the AutoComplete memory and the Contacts folder side by side, so you can see exactly which list a suggestion comes from.
▶️ Watch Full VideoBecause AutoComplete remembers addresses, not contacts. If you sent to or replied to that address even once, it can live in the AutoComplete cache for years. The suggestion has nothing to do with your address book, and deleting the contact (if one exists) will not remove the suggestion - you remove the suggestion itself, as our single-entry guide shows.
Outlook's To-field drop-down is driven by the AutoComplete list, and a contact only joins that list after the address has been used in a sent email at least once. Save the contact, send them one short mail, and from then on the name is suggested. The Address Book and Search People box do read contacts directly if you prefer that route.
About 1,000 entries in modern classic Outlook. When the list is full, the least-used older entries quietly fall off as new ones arrive. That limit is one more reason important addresses belong in Contacts, which has no such small cap and is backed up with your mailbox.