Provided by: courier-mta_0.66.1-1ubuntu3_i386 bug

NAME

       localmailfilter - Local mail filtering

SYNOPSIS

       echo /usr/bin/maildrop >/etc/courier/maildropfilter

       mkdir $HOME/.mailfilters

       vi $HOME/.mailfilters/rcptfilter $HOME/.mailfilters/rcptfilter-ext

       vi $HOME/.mailfilters/smtpfilter $HOME/.mailfilters/smtpfilter-ext

       chmod 700 $HOME/.mailfilters

       chmod 600 $HOME/.mailfilters/*

DESCRIPTION

       The maildrop mail filter can be used by the Courier mail server as a
       mail filtering engine, rejecting unwanted mail on a per-recipient
       basis.

       The actual filtering interface used by the Courier mail server does not
       really require that maildrop must be used as a mail filtering engine,
       it just so happens that maildrop has a compatible interface that can be
       used right out of the box. The following brief information can be used
       to craft a homebrewed mail filter to take maildrop's place.

       The local mail filter only works for addresses that correspond to local
       accounts. This filtering is not used if the recipient is a remote
       address on another mail server. The local mail filter is disabled by
       default. To enable local mail filtering you will need to initialize the
       /etc/courier/maildropfilter configuration file to contain the pathname
       to your local mail filter.

       Local mail filtering is performed in two distinct phases:

       Recipient filters
           When the Courier mail server receives an address naming a local
           mail recipient, the local mail recipient's mail filter is executed
           before the Courier mail server acknowledges the address. The local
           mail filter tells the Courier mail server whether to: A) accept
           message unconditionally - the message is whitelisted; B) reject the
           message unconditionally - the Courier mail server tells the other
           mail server that the recipient address is invalid; or C) accept
           this recipient, but run the content mail filter, once the message's
           contents are available.

       Content filters
           After receiving the contents of the message, the mail filter is
           executed again for any recipients whose recipient filters used the
           third option. The content filter can now examine the contents of
           the message, and indicate whether the message should be accepted or
           rejected. Content filtering is not available for alias addresses.

       It should be noted that mail filtering is executed as an integral part
       of receiving a message from a remote mail server. If the message is
       rejected, the Courier mail server refuses to accept the message for
       delivery.

       The local mail filter will be invoked as follows:

           HOME=$HOME FILTER -D uid/gid -M filter

       The local mail filter will NOT be invoked as root, so if it needs to
       access files in the recipient's account, it must be installed setuid to
       root (as maildrop is installed by default).

       "uid/gid" is the recipient account's system userid and group id,
       respectively. The recipient account's home directory is placed in the
       HOME environment variable, prior to running FILTER, and "filter" is set
       as follows:

       rcptfilter
           The mail filter is invoked initially when the remote mail server
           specifies this address as a recipient.  FILTER should terminate
           with one of the following exit codes: 0 - this sender is
           acceptable; 99 - this sender is acceptable, but I want to run the
           content filter for this the message; any other non-zero exit code -
           the sender is not acceptable, reject the message.

       smtpfilter
           If FILTER terminates with exit code 99, FILTER runs again with this
           parameter set to the word smtpfilter. FILTER will be invoked once
           the message has been received from the remote mail server, but not
           yet acknowledged. If FILTER terminates with a non-zero exit code,
           the message is rejected. If FILTER terminated with the exit code of
           zero, the message is accepted.

       rcptfilter-ext, smtpfilter-ext
           If the recipient created sub-addresses - see dot-courier(5)[1] - a
           dash followed by the subaddress "ext" is appended to the name of
           the filter.

       rcptfilter-alias-ext
           This is how FILTER gets invoked if the address is a locally defined
           mail alias (ext is the alias name).

       The rcptfilter invocation must terminate with a zero exit code when the
       message originates from a mailing list or any other source that should
       be considered as "whitelisted". This filtering model does not fit very
       well with some mail transfer protocols, so unless trusted sources are
       explicitly declared to be whitelisted, there is a remote possibility
       that the recipient will be removed from a mailing list because of a
       poorly-written mail filter from some other recipient of the same
       message. The 0 return exit code (which is the implied default if no
       mail filtering is installed) protects the recipient from being
       adversely affected, in any way, by anyone else's mail filter.

       The mail filters may print a diagnostic message before rejecting a
       message. The diagnostic message will be returned to the sending mail
       relay, where possible.

       The mail filters inherit environment variables that describe the
       incoming mail. The following environment variables are provided by
       default:

       SENDER
           The return address on the message.

       TCPREMOTEHOST, TCPREMOTEIP
           When the message is received via ESMTP, these variables specify the
           remote IP address and the corresponding hostname. Hostname is empty
           if the IP address does not have a reverse DNS record, or is set to
           "softdnserr" if there was a temporary failure while looking up this
           IP address.

       BLOCK2
           The default the Courier mail server configuration sets this
           environment variable if the remote IP address is listed in an
           unsecured relay blacklist. See /etc/courier/esmtpd for more
           information. Other environment variables may also be available. For
           mail received via ESMTP, environment variables are usually set in
           the /etc/courier/smtpaccess configuration file.

   maildrop implementation
       Maildrop implements this mail filtering API as follows:

       $HOME/.mailfilters
           This directory contains the filtering recipes. This directory, and
           its contents, cannot have any group or world permissions.

       smtpfilter*, rcptfilter*
           These mail filtering recipes directly correspond to the events
           defined in the previous section. Maildrop's "import" statement can
           be used to gain access to the environment variables (these mail
           filters are executed in maildrop's embedded mode). The mail
           filtering recipes can set the EXITCODE variable appropriately
           before terminating, in order to accept or reject the message.

       See maildrop(1)[2] for more information.

   Filtering mail to aliases
       The /etc/courier/aliases configuration file is used to mail aliases,
       see makealiases(8)[3]. The system administrator may set aside a
       reserved local account that will be used to specify a local mail filter
       for messages addressed to aliases. The configuration file
       /etc/courier/aliasfilteracct specifies the home directory of the mail
       account that will be used to filter alias recipients.

       For example, if /etc/courier/aliasfilteracct contains /home/admin, then
       the Courier mail server runs the mail filter as follows:

           HOME=/home/admin FILTER -D uid/gid -M rcptfilter-alias-name

       Here, "uid/gid" is owner uid and gid of the specified directory NOTE:
       "name" is a fully qualified address, and the local aliases listed in
       /etc/courier/aliases do not typically include the domain name. If
       defines an alias called "system", for example, the -M option will
       probably be "system@example.com", if example.com is the contents of
       /etc/courier/me configuration file.

       Unfortunately, currently it is not possible to specify content filters
       (a.k.a. smtpfilters) for aliases, only recipient filters.

FILES

       /etc/courier/maildropfilter
           Local mail filtering engine.

       /etc/courier/aliasfilteracct
           Account that is used to filter mail to aliases.

SEE ALSO

       courierfilter(8)[4], maildrop(1)[2].

AUTHOR

       Sam Varshavchik
           Author

NOTES

        1. dot-courier(5)
           [set $man.base.url.for.relative.links]/dot-courier.html

        2. maildrop(1)
           [set $man.base.url.for.relative.links]/maildrop.html

        3. makealiases(8)
           [set $man.base.url.for.relative.links]/makealiases.html

        4. courierfilter(8)
           [set $man.base.url.for.relative.links]/courierfilter.html