When you don’t really know how to call something, it’s hard to find it. I tried “mail.app saved e-mail addresses” or “mail.app autosave e-mail address” and still Google gave not the required article. At least I thought.
An annoying thing about Mail is that it saves e-mail addresses to which you previously sent some mail to. It isn’t necessarily annoying, but it is when you can’t change the format. Jack Nickleback might be in your address book, but you might have replied to him while he’s sending with the nickname “Jacky Nickly 98347 <jacky @hotmail.com>” and you really don’t want that to appear in your “To:” field. You just want to send it to the format in which you specified his name in your address book. Simple and clean.
Now there is a feature – luckily – to change this, but I expected an option to change those saved passwords. It isn’t. You can go to Window > Previous Recipients and that’s the list of – d’uh – the e-mail addresses to which you have previously sent an e-mail. Bad thing is: it doesn’t show up the name with which they appear in your e-mail message. What you have to do, is remove the names from the list. They will still appear as you type, since they are in your address book and mail searches the Previous Recipients list and then the Address Book.
I found this in an article from someone who does read the manual, so I don’t have to, but it is on the Apple Support Website too.