When you create a new email and select an email address from the drop down history that is also the recipients second email address in their contact, the email is sent to the recipients
first email address.
Outlook Contact -John Doe
Email - 1 -
Email - 2 -
If I select
email@example.com from the dropdown history in outlook and send, when I look in my sent items, it went to
firstname.lastname@example.org - recipient has confirmed that it wet to the wrong address.
Backend is Exchange 2010