localmailfilter - Local mail filtering
echo /usr/bin/maildrop >/etc/courier/maildropfilter
vi $HOME/.mailfilters/rcptfilter $HOME/.mailfilters/rcptfilter-ext
vi $HOME/.mailfilters/smtpfilter $HOME/.mailfilters/smtpfilter-ext
chmod 700 $HOME/.mailfilters
chmod 600 $HOME/.mailfilters/*
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
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:
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.
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
is installed by default).
" is the recipient account's system userid and group
id, respectively. The recipient account's home directory is placed in the
environment variable, prior to running FILTER
" is set as follows:
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.
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.
If the recipient created sub-addresses - see
dot-courier(5) - a dash followed by the subaddress
"ext" is appended to the name of the filter.
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
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:
The return address on the message.
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.
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 implements this mail filtering API as follows:
This directory contains the filtering recipes.
This directory, and its contents, cannot have any group or world
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
(1) for more information.
The /etc/courier/aliases configuration file is used to mail aliases, see
(8). 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
probably be "firstname.lastname@example.org", 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.
Local mail filtering engine.
Account that is used to filter mail to