I have been caught by unexpected autocompletion many times. Of course, I am responsible for double-checking the recipient before sending, but it can be an easy mistake to make when in a hurry. I suggest allowing the user to customise the autocompletion so that it matches their working style better. Some issues I've noticed are: i) Multiple matches: "maggie" matches: <maggie@example.com> (matching on address) Maggie Surname <megsurname@hotmail.com> (matching on name) "ad" matches: Ada Surname <adasurname@example.com> Allan Dyer <adyer@example.com> ii) Unexpected completion: With "Automatically add outgoing email addresses to my Collected Addresses" turned on, replying to a message will add the address to the address book, but the user may not be aware of the actual email address, or the conflicts it causes with existing addresses in the address books. The user types a fragment that previously only had a single match, and sends the message before realising there was a new match selected. The extra controls I suggest: Match Name / Match Address / Match Either Name or Address Include / Exclude option for each address book Reason: The user can select which group of contacts are used for autocompletion. If Collected Addresses is excluded by default, then there is less chance of an unexpected autocomplete. Minimum match length (characters) Reason: One or two character matches are much more likely to have accidental matches. Thanks.