Summary of features:
* Act as an external mailer for sendmail using LMTP(RFC2033)
* No temporary files -- just on memory processing
* Simultaneous DNS(MX/A RRs) query for multiple destination domains
* Simultaneous SMTP transaction to multiple destination hosts
  (up to limitation on number of file descriptors)
* MX piggybacking with a RR base (not with sequence of MX RRs)
* ESMTP Pipelining (RFC2197)
* SMTP connection caching (for deriveries of copies of a message)
  set -u option to enable
* Failure caching (through running queue operation)
* Fallback MX host can be specified
* Static routing map support (see -M option in OPTIONS file)
* IPv6 support

Known bugs:
* ESMTP DSN(Delivery Status Notification; RFC1891) information will be
  ignored if DSN is not supported by a destination SMTP server
* 8bit message will be sent as is to an SMTP server which does not support
  ESMTP 8BIMIME (RFC1652) without 8to7 conversion
* Checkpointing feature of sendmail is not effective with smtpfeed mailer