SMTP is the standard mail protocol that is used on the Internet. This protocol defines these mailers.
smtp provides regular 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 24, What's New With Mail Services (Reference).