Q: How can I stop Mail from automatically adding email addresses to Contacts?

Mail does not automatically add addresses to Contacts.

It adds them to the previous recipients list (Window>Previous Recipients)

I'm getting a little tired of this myself.  Everyone is so fond of saying that this doesn't happen, but it's happening to me as well.  Whenever I send a message using the OS X Mail app, I suddenly find that same e-mail address in my contacts.  It's been happening for months now, and I first noticed it when I was on Mavericks.  It's still happening on Yosemite. 

Send Apple feedback. They won't answer, but at least will know there is a problem. If enough people send feedback, it may get the problem solved sooner.

Feedback

Office 365 and Exchange accounts have a "Suggested Contacts" feature that has caused this problem for me.  When I send messages through my Exchange account to email addresses not in my Exchange contact list (even if they are in a contacts list under another account), Exchange automatically adds their email address to my Exchange contacts.  While there is a setting to disable this if I was using the Windows version of Outlook 2010/2013, I'm using Apple Mail, which means I cannot correct the problem.  There doesn't appear to be a setting through my Office 365 online account that controls this "feature", either.

To address the issue for now, I have to manually clean up the Exchange contact list periodically.  Thus in my case, this is a Microsoft problem, rather than an Apple problem.  That may be why so many answers to this question are, "Mavericks/Yosemite does not do this".

That might be what my problem is, as I'm using Exchange.  Thanks for the input, it ***** that there's no better solution.

This seems to be isolated to using Mail on Yosemite when piping the email through an Office365 account. For some reason those added emails do not sync to the iPhone (would be another mess) and hence are only created on the Yosemite Mail side, I can't see those emails/contacts on the Office365 side, so the problem seems to be rather on the Yosemite Mail side displaying something that iOS or Office365 doesn't.

That makes sense, I just wish there was a way to shut it off.

For me all my contacts are stored in iCloud. This issue was driving me nuts, but I just tried it.

It is an exchange issue. The steps to resolve.

     • Open System Preferences

     • Open Internet Accounts

     • Select your Exchange Account

     • Turn Off Contacts for Exchange

You should now see those automatic generated contacts gone.

I have had the same issue. I just went into into my Exchange Account Settings on my iPhone and turned off the Contacts toggle. This seems to have worked. Fingers crossed.

I was on the phone with Apple support for over an hour and the rep couldn't help with this - it took 2 minutes with the solution from Dan Desimoni!