Name: smtp_backend
Type: STRINGLIST
Allowed-Values: host sendmail
Default-Value: sendmail
Last-Modified: 3.1.4

The SMTP backend to use for sending email.

The \"host\" backend sends message submissions via a TCP socket to the SMTP
host defined in the config option smtp_host.

The \"sendmail\" backend forks the Cyrus process into the executable
defined in the config option sendmail.  The executable must accept \"-bs\"
as command line argument, read from stdin and must implement the minimum
SMTP protocol as defined in section 4.5.1 of :rfc:`5321`.

If the SMTP EHLO command reports AUTH (:rfc:`4954`) as a supported
extension, then the MAIL FROM command includes the AUTH parameter, with its
value set to the name of any authenticated user which triggered the email.
The AUTH parameter is omitted if the user is unknown to the calling
process.

If the directory *configdirectory*/log/smtpclient.\ *smtp_backend* exists,
then telemetry logs for outgoing SMTP sessions will be created in this
directory.
