RE: How to use Procmail to remove messages from server after x numberof

At 08:36 2011-03-01, Komal Tagdiwala (ktagdiwa) wrote:
> You can use email clients like Thunderbird which can be configured to
> PURGE messages older than X number of days.
I'd presume that the OP is doing IT for an office or a small ISP, and 
wants to limit the amount of stored mail.  It would be difficult to 
consistently enforce the use of a specific email client, and ensure 
that they're all configured to purge messages, etc.
Besides, there's a HUGE failing in that approach: if someone doesn't 
log in for a month, their client software isn't connecting and 
purging the messages at all.  So, the ACTIVE users might purge their 
messages with client configured to do so, but the INACTIVE users 
continue to have email pile up.
An ISP I adminned for didn't much care how much or how long you 
stored your email.  Beyond a certain amount of disk use, it was all 
billable storage - if someone wants to leave their email on the 
server indefinately, then let them - it would reflect in their 
bill.  Obviously, that approach won't work for a corporate environment.
I question the logic of deleting messages > 10 days old though - in a 
business environment, there's got to me older messages that need to 
be preserved, and in a customer/user environment, I'd flee the 
service the instant they purged my email (ok, nevermind that I don't 
entrust my email to other people's servers anyway).

As to actually purging the messages, you need to determine if the 
messages are folder or mailbox format.  The former would be easier to 
delete individual messages from, but isn't something you'd use 
procmail for.  You also need to know whether the users might have 
processes manipulating their mailboxes (shell access, their own 
procmail), or if it's just the LDA + POP.
You should be able to lock the mailbox and move it, unlock it, then 
newly arriving email (with is 0 days old) shows up into the normal 
mailbox while you're reprocessing the old mail.  You reprocess the 
moved mailbox into a new file, discarding messages that meet your 
criteria (I'd use the date in the From_ line, FWIW), and then when 
done, lock the main mailbox file, concatenate it (if there's anything 
in it) to your reprocessed file (no per-message examination 
necessary), and replace the main mailbox with the reprocessed one, 
then unlock it.  During this time, you won't have limited access for 
arriving mail for more than a few cycles.
Most of what you need to accomplish is shell scripting.  The recipe 
for procmail to essentially /dev/null messages over 10 days old is 
near trivial.  This here is untested:
# Extract the date from From_, as well as the current time.
# both represented in seconds from epoch.
# you're on your own if there isn't a valid date in the From_ header
# (but WHY would that happen?)
:0
* ^From[        ]+[^    ]+ +\/[^        ].*
{
        FROM_SECS=`date --date "$MATCH" +%s`
        CURRENT_SECS=`date +%s`

        # establish your threshold, in seconds
        # 10 * 24 * 60 * 60 = 864000
        THRESHOLD_SECS=864000

        # use scoring for primitive math evaluations.
        # The current time, minus the original message time is how old the
        # message is (which should be a positive value).  Subtract our
        # threshold from that, and if the threshold is greater than the
        # message age, then this evaluates with a negative result, and we
        # don't discard.  If the threshold is less than the message age
        # (i.e. > 10d), then we have a positive result and evaluatte true
        # -- discarding the message as our action.
        # you could instead store it to a separate mailbox to be handled by
        # the script which invoked procmail.  If you do, be sure to use the
        # locking flag.
        :0
        * $ $CURRENT_SECS^0
        * $ -$FROM_SECS^0
        * $ -$THRESHOLD_SECS^0
        /dev/null
}


However, there's the issue of whether various processes properly 
lock, or what they do to the mailbox while they're handling it - your 
POP server for instance may make a copy of the mailbox and DELE from 
their client software may purge from that copy.  Same goes for a 
shell based MUA, and who knows about web-based access.  If you 
process the user mailbox while it's being manipulated by another 
process (or rather, while most of its content has been copied), you 
could have very unpredictable results.
A "simple" software upgrade of a server package could complicate 
things as well.
---
 Sean B. Straw / Professional Software Engineering

 Procmail disclaimer: <http://www.professional.org/procmail/disclaimer.html>
 Please DO NOT carbon me on list replies.  I'll get my copy from the list.

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