sendmail, 4th Edition

FastSplit

Suppress MX lookups on initial submission V8.12 and later

When sendmail expands an alias (The aliases(5) File on page 460), as when using aliases to send to a mailing list, sendmail sorts the list of new recipients by host. Normally, that list of hosts is then sorted by MX record rather than hostname. After sorting, the new MX-sorted list is split by sendmail into multiple envelopes.

Envelope splitting (also called cloning) creates multiple envelopes when there was originally only one. Each new envelope contains fewer envelope recipients. Normally, all these envelopes are delivered in parallel for delivery efficiency.

This process is intended to create delivery efficiencies, but on high-traffic machines, it can actually create slowdowns because:

• Converting hostnames to MX records requires a DNS lookup for each hostname.

• Large lists can lead to far too many parallel deliveries.

Although the FastSplit option can be used to both eliminate MX lookups and limit the number of parallel deliveries, these two functions cannot be decoupled. The FastSplit option is used like this:

O FastSplit=num                    ← configuration file (V8.12 and later)
-OFastSplit=num                    ← command line (V8.12 and later)
define(`confFAST_SPLIT',`num')     ← mc configuration (V8.12 and later)

Here, num is of type numeric. If it is negative, non-numeric, or zero, the normal behavior of sendmail is allowed (hosts are sorted by MX record, and there is no limit on parallel delivery). The default is one.

If num is greater than zero, it ...