Somehow, contacts are being added automatically to my Mac address book. They contain only an email address, which suggests that a mail program is creating these entries automatically. The only email clients I use are Mac Mail on the desktop and the Mail app on iPhone. I can't find any setting in either app that would cause this behavior.
New entries are even being created for email addresses that are already associated with an existing contact. And if I delete them, they just come back again (though I can't tell when exactly that happens).
Mac Mail is set up for an iCloud account and an Exchange account. But the entries that are being added do not exist in the Exchange directory.
From Mac Mail, I tested both sending and receiving to/from an address not in my address book, but I did not observe an entry being created automatically.
Any idea what else might be causing this behavior?!