This is trickier than it should be.
Address Book assumes that the first email address listed for a contact is the primary, main, default email address. Makes sense, right? And yet you may still be seeing a secondary address appear by default when you try to send email to that person in Mail.app! How annoying!
The explanation is that Mail tries to guess which address you are most likely to want based on past usage. So if a secondary email address has been used a lot recently, it may over-ride the preference set in Address Book.
Yes, this is stupid. It’s a classic case of a computer trying to be so clever and helpful that it backfires. (Remember Microsoft’s “Office Assistant”? Mr. Clippy?)
Fortunately, you can erase Mail’s memory of an address. Do this, and sanity will be restored. So to make sure that an email address is respected as the primary:
- In Address Book, edit the contact and re-arrange their email addresses, so they are in the desired order, i.e. the primary email you want to use is first.
- In Mail, go to Window > Previous Recipients, scroll to the contact in question, and remove all their entries from the list.
The next time you compose an email to that person, Mail will go back to Address Book to check for your preference.