Email Setting-Change:From,Subject,Date,To.

Every time i send a email were the Subject, Date, From, and TO, i can not edit To, there is a persons name that is also highlighted and i can not delete that person and their email address, eg
From:
Subject:
Date:
To:
Their name is there and when highlighted their email come up.
I am no longer with this person and want to delete their details,
does this mean ever time i send an emails it also goes to this person?
I have looked through mail setting and cannot find how to delete, any advice would be appreciated.