Reader Fran Drakes has an issue with autofill and Mavericks’ Mail app. She writes:
My husband has three email addresses. I almost always want to send to his Mac.com address, and for years, that’s what Mail automatically used when I typed his name. But when I upgraded to Mavericks (at least that’s what appears to have been the trigger), Mail decided that I really wanted to send to his Yahoo address. So every time I add him to an email, I have to manually change the auto-filled address to his Mac.com address. Is there a way to force Mail to always use the same address?
I sympathise. With Mavericks’ Mail, auto-fill will choose the first alphabetical address, regardless of the order it appears in a contact’s card. For example, in the case of bubba1@example.com, bubba2@example.com, and bubba3@example.com, bubba1 will be auto-fill’s choice. So let’s take it out of auto-fill’s hands and tell Contacts and Mail exactly what we’d like to have happen.
Launch the Contacts app, select your husband’s name in your list of contacts, and choose File > New Group From Selection. This will create a new untitled group with a highlighted name. Rename that group something like “hubby.”
Now choose Edit > Edit Distribution List and in the sheet that appears select your “hubby” group. You’ll see your husband’s name along with the email addresses that appear on his contact card. Select his Mac.com address so that it turns black and click OK. What you’ve essentially told Contacts is that within this group (of one) the Mac.com address is the default.

Choose the preferred email address in the distribution list sheet.
Return to Mail, create a new message, and in the To field enter “hubby.” When you press the Return key, that address should change to his name and use his Mac.com address. At a later time, should you wish to send to a different email address, enter his name rather than “hubby” and then choose the address you wish to send to from the auto-fill list that appears.