One of the niggling problems of SMTP in the modern world (at least for us) is the mixed address problem, the fact that at DATA time your answer applies to all recipients. It would be much more convenient if all email messages had only a single recipient; then you could always apply just that recipient's content filtering views and enable much more rejection at SMTP time. Which leads to the question: how many recipients does an average message here have, especially inbound messages?

(Inbound messages are the most interesting ones, because those are the ones that all of our anti-spam stuff is applied to.)

Today, I decided to answer that question for our external MX gateway. The answer turns out to be that the overwhelming majority of email has only one recipient. The stats break down like this:

1 recipient	93%
2 recipients	3.6%
3 recipients	1.2%
4 recipients	0.6%
5 recipients	0.4%
6 recipients	0.2%
10 recipients	0.2%

(I think I'll stop there.)

This is from 89 days of logs, totaling 1.29 million messages received. It counts only actual accepted recipients so some of these messages may have had some of their RCPT TOs rejected already (I suspect that this is not a really big factor but I haven't looked).

The largest number of (accepted) recipients for a single message is 82 recipients (one messages). There are a similar handful of other messages with large recipient counts. Interestingly the largest 'large' message count is for 20 recipients (but it's still only 0.09% of all messages). There seems to be a hard break at 20 recipients; only 98 messages out of the 1.29 million had more recipients than that.

This has been interesting. Before I did these stats I would not have expected single-recipient messages to be so totally dominating (even though I'm familiar with things like VERP that strongly bias some traffic towards that). Possibly much more of our inbound email is mailing lists (including spam lists) than I expect.

Sidebar: detailed message counts for 7-20 recipients

This actually forms an interesting pattern so I'm going to give you the raw data:

   cnt   recipients
  1210   20
   641   19
   372   18
   184   17
   136   16
   113   15
   153   14
   173   13
   289   12
   820   11
  2081   10
  1428   9
  1568   8
  1925   7
  2156   6

(for 2-7 there is a steady dropoff.)

My guess is that a bunch of mailing list software really prefers to cut things at nice even (small) numbers of recipients.