[exim-dev] [Bug 1406] New: SMTP backoff based on queue size

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http://bugs.exim.org/show_bug.cgi?id=1406
           Summary: SMTP backoff based on queue size
           Product: Exim
           Version: N/A
          Platform: All
        OS/Version: Linux
            Status: NEW
          Severity: wishlist
          Priority: medium
         Component: Queues
        AssignedTo: nigel@???
        ReportedBy: paul.osborne@???
                CC: exim-dev@???

Under certain circumstances I find it would be useful to have Exim periodically
back off accepting SMTP connections based on the number of items in the mail
queue waiting to be processed by other discrete services in the system (ie
AV/spam etc.

Whilst this is similar to the existing options such as:

check_spool_space
check_spool_inodes
to check storage usage

smtp_load_reserve
to check system load

these depend on the OS being able to report these details, in this case though
Exim itself can query its own queue length exim -bpc periodically which does
mean that control can be based on the number of items of mail rather than some
other arbitrary system resource.

Perhaps something then like:

smtp_backoff <highwatermark> <lowwatermark> <interval>

where exim stops accepting mail at the high number, accepts mail again at the
low number, and checks the queue size at the appropriate interval. Whilst not
accepting email a temporary 4XX deferal series message should be issued so as
not to scare off upstream servers completely away.

Many thanks.

Paul


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