I am having a similar problem, but the email that I am using in not an apple email.
My wife was sending an invoice via MYOB to client, and somehow, now the email address she was sending FROM, will say that clients name when I try to send email TO it, from within Apple Mail. This does not happen if I use AirMail and it happens on all our computers, so it appears to be tied to the contacts. IF I look in our contacts both locally and on iCloud, I can not see this clients name anywhere, and even so, I would not think that MYOB could modify Apple Contacts.
it is a real mystery. The email get sent from our address, it just changes the recipient's name to this clients name.
Would love to stop this happening.
Craig
PS, this is on 10.10.4, and the most current Apple Mail / Contacts apps.