It depends upon the program you are using, whether it's an installed E-Mail Client like Outlook, Thunderbird, Incredi-Mail, etc., or an online/Webmail app like Hotmail.com, Outlook.com, Live.com, Gmail.com, Yahoo.com, etc. There should be a feature in whatever that addresses Contacts or Addresses, opening that should give access to managing your list. Also, if an address in the To: line isn't needed it can be deleted for that message only.