When you get an email from a yahoo group list, for example, it might say it was sent from yourgroup@yahoogroups.com on behalf of the person who actually sent it. Both addresses will be included in the message. This is done in SMTP with the SENDER header. If the Sender header is present, the email client should identify the mail as being "from" the sender you specify in the SENDER header (they are "the responsible mail submitter"), on behalf of the from-address that you specify in the FROM header....