I assume that you're using an email program like Windows Mail, Outlook Express or MS Outlook and that you're not web based.
If this is so and you have multiple accounts on your email prog (i.e. more than one email address) you must send the email with the account in which the mail was created. If you compose an email whilst one of your accounts is the default sender you must send it with that account and not the other. You will always have to delete manually if you've once tried to send it with the wrong account.
Hope that makes sense!