I have lost the automatic autofill function for sending email addresses for new messages in Yahoo Mail using Yosemite. The Yahoo Mail account has access to iCloud and Yahoo Contacts. I used to be able to type in a last name after “TO” in the Compose and get the email address, but now the address does not come up.
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Gene
Not sure if you mean in Yahoo's web-based mail interface or in Apple's Mail app. If the latter, then I would check in the Contacts app under Contacts, Preferences, Accounts and make sure your Yahoo account is there and enabled.
The problem is in Yahoo Mail not Apple Mail
Gene: So in the web-based interface then? Is that what you mean? (You can also use Yahoo mail in the Apple Mail app).
If that is the case, then I'm afraid I don't have an answer for you. I don't use Yahoo mail -- you'll need to ask them for help (I don't think they offer support) or find an expert at Yahoo mail to help, sorry.