As discussed in Accept Bounces Only for Real Users, there is now a
loophole that prevents us from catching bogus Delivery Status Notification sent to system users and aliases, such as
postmaster. Here we cover two alternate ways
to ensure that bounces are only accepted for users that actually
send outgoing mail.
The first method is performed in the acl_rcpt_to ACL. Here, we check that the recipient address corresponds to a local mailbox:
# Deny mail for users that do not have a mailbox (i.e. postmaster,
# webmaster...) if no sender address is provided. These users do
# not send outgoing mail, so they should not receive returned mail.
#
deny
message = This address never sends outgoing mail. \
You are responding to a forged sender address.
log_message = bogus bounce for system user <$local_part@$domain>
senders = : postmaster@*
domains = +local_domains
!mailbox check
Unfortunately, how we perform the mailbox
check will depend on how you deliver your mail (as
before, we extract the portion before the first “=”
sign of the recipient address, to accomodate for Envelope Sender Signatures):
If mailboxes map to local user accounts on your server, we can check that the recipient name maps to a user ID that corresponds to “regular” users on your system, e.g. in the range 500 - 60000:
set acl_m9 = ${extract{1}{=}{${lc:$local_part}}}
set acl_m9 = ${extract{2}{:}{${lookup passwd {$acl_m9}{$value}}}{0}}
condition = ${if and {{>={$acl_m9}{500}} {<${acl_m9}{60000}}} {true}}
If you deliver mail to the Cyrus IMAP
suite, you can use the provided mbpath
command-line utility to check that the mailbox exists.
You will want to make sure that the Exim user has
permission to check for mailboxes (for instance, you may
add it to the cyrus group:
# adduser exim4 cyrus).
set acl_m9 = ${extract{1}{=}{${lc:$local_part}}}
condition = ${run {/usr/sbin/mbpath -q -s user.$acl_m9} {true}}
If you forward all mail to a remote machine for delivery, you may need to perform a Recipient Callout Verification and let that machine decide whether to accept the mail. You need to keep the original envelope sender intact in the callout:
verify = recipient/callout=use_sender
Since in the case of locally delivered mail, this mailbox check duplicates some of the logic that is performed in the routers, and since it is specific to the mail delivery mechanism on our site, it is perhaps a bit kludgy for the perfectionists among us. So we will now provide an alternate way.
You probably have a router named
system_aliases or similar, to redirect mail
for users such as postmaster and
mailer-demon. Typically, these aliases are
not used in the sender address of outgoing mail. As such, you
can ensure that incoming Delivery Status Notifications are not routed
through it by adding the following condition to the router:
!senders = : postmaster@*
A sample aliases router may now look like this:
system_aliases:
driver = redirect
domains = +local_domains
!senders = : postmaster@*
allow_fail
allow_defer
data = ${lookup{$local_part}lsearch{/etc/aliases}}
user = mail
group = mail
file_transport = address_file
pipe_transport = address_pipe
Although we now block bounces to some
system aliases, other aliases were merely shadowing existing
system users (such as “root”,
“daemon”, etc). If you deliver local mail
through the accept driver, and use
check_local_user to validate the recipient
address, you may now find yourself routing mail directly to
these system accounts.
To fix this problem, we now want to add an additional condition in the router that handles your local mail (e.g. local_user) to ensure that the recipient not only exists, but is a “regular” user. For instance, as above, we can check that the user ID is in the range 500 - 60000:
condition = ${if and {{>={$local_user_uid}{500}}\
{<{$local_user_uid}{60000}}}\
{true}}
A sample router for local delivery may now look like this:
local_user:
driver = accept
domains = +local_domains
check_local_user
condition = ${if and {{>={$local_user_uid}{500}}\
{<{$local_user_uid}{60000}}}\
{true}}
transport = transport
Beware that if you implement this method, the reject response from your server in response to bogus bounce mail for system users will be the same as for unknown recipients (550 Unknown User in our case).