SMTP-host-choice "free":
The user will be allowed to choose any SMTP-server for his mail to be
send. This setting should be handled with care, since users could abuse
your application-server to send spam.
SMTP-host-choice "domain" (default):
The user will be allowed to choose a SMTP-server within the domain of the
mailbox-host (for example if the user has a mailbox host "pop.domain.tld",
he will be able to choose a SMTP-server within the domain "domain.tld").
This considerably reduces the the users chance to send spam by dint of
your server.
SMTP-host-choice "none":
The user can only use the SMTP-host you allow. With this setting you also
have to specify the properties "forced_smtp_host_name" and
"forced_smtp_host_port".