Wandering Thoughts archives

Looking at how many external recipients inbound email goes to

My data on how many recipients our average inbound email has is in practice incomplete. It's quite possible for a single address here to expand into multiple destinations; some addresses are mailing lists and some people just forward their email to more than one place. So an interesting companion question is how many external recipients a typical email has. To make this more applicable to what I'm interested in, I'm looking at this only for email from the outside world.

As before, this covers 89 days of logs (but because it's a slightly different 89 days, the stats don't necessarily match up exactly). The first number is that out of 1.3 million inbound emails, only 30% had any external recipients at all; the remaining 70% went (directly or indirectly) only to internal recipients. The recipient count breaks down this way:

1 recipient	91.7%
2 recipients	4.6%
3 recipients	1.9%
4 recipients	0.5%
5 recipients	0.4%

As you might expect in an environment with mailing lists, some messages had very high external recipient counts. The champions were emails with between 247 and 266 external recipients, all of which seem to have been messages to department-wide mailing lists (which of course go to a whole lot of people who forward their email to outside addresses). But there weren't very many such emails; only 0.4% of the messages had 10 or more external recipients.

Unlike the inbound email case there don't seem to be any particular pattern for significant numbers of external recipients. This is what I'd expect given that the mapping between the number of inbound recipients and the number of external recipients is a fairly random one (since it depends on exactly who the email goes to).

Looking at how many recipients our average inbound email has

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.