Provided by: courier-mta_0.66.1-1ubuntu4_amd64 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