Apple Mail autocomplete problem

I just added a new email account to my apple mail reader and ever since then I have had a problem. When I start typing an email address into the "to" field, my computer tries to autocomplete the address which is fine. But the problem is that if I try to type in an email address that my computer has never seen before and I get to the end of the email address and hit return or comma, the "to" field does not capture what I just typed in. It converts what I typed into the last Autocomplete email address that it THINKS I wanted.