SMTP is the standard mail protocol that is used on the Internet. This protocol defines these mailers.
smtp provides regular (historic-style) SMTP transfers to other servers
esmtp provides extended SMTP transfers to other servers
smtp8 provides SMTP transfers to other servers without converting 8-bit data to MIME
dsmtp provides on-demand delivery by using the F=% mailer flag. Refer to "Changes to the MAILER() Declaration" and "New Delivery Agent Flags" in Chapter 27, What's New With Mail Services (Reference).