Why does OS X Mail fail to recognize that an e-mail address belongs to an existing contact? In OS X Mail, when I enter the name of a person to whom I have sent a previous e-mail, I'm often presented with a suggested e-mail address, corresponding (roughly) to the most recent frequently sent-to person matching the entered name, or part of a name. These suggestions make sense, and I realize that they are coming from my previous recipients list, but what I don't understand is this: often, though the proposed address is associated with a contact in my address book (and is marked as such in the Previous Recipients list), it is not recognized as being associated with the contact in the sent message. For example, if I have a contact, say:

Joe Stack
joe@aaa.com
joe@bbb.com
joe@ccc.com


and I have been corresponding a lot recently with him at joe@bbb.com then when I begin typing his name in the To field rather than proposing 

Joe Stack


along with context menu containing a list of all of the e-mail addresses from his contact information, I get

Joe Stack &lt;joe@bbb.com&gt;


with no alternate choices of e-mail and no connection to his contact. In fact, in the context menu for this address, I'm given the option to Add To Address Book rather than Show Contact Card (and if I add it, it is added as a distinct additional entry, so that now there are two entries with joe@bbb.com as an e-mail address).

What could be causing Mail to fail to recognize that the joe@bbb.com address is for an existing contact? This behavior has been going on for some time (at least since 10.6) and continues in Mountain Lion.
I was having the same issue. To resolve it, I went into the previous recipients list and removed all entries of the offending address, as well as any other email addresses associated with that person. From that point forward, when composing an email it correctly pulled the Address Book entry and not the independent address.