Hi
This is what Yahoo says about it . . . . .
Can I disable or delete email addresses from the Autocomplete feature?
ID: SLN4642 Refers to: Mail
Description
The Autocomplete dropdown is suggesting wrong or old email addresses when a contact's name is entered into the address fields in Yahoo! Mail.
Resolution
Autocomplete is designed to suggest email addresses based on contacts that you have sent messages to in the past, even if they are not stored in your address book. Suggestions are sorted in order or relevance, rather than in alphabetical order.
Unfortunately, it is not possible to disable the Autocomplete feature or
manually delete contacts from the Autocomplete list. We apologize for any inconvenience this may cause.
