[Mailman-Users] how are large distribution lists handled

Brad Knowles brad at stop.mail-abuse.org
Fri Jul 4 07:39:35 CEST 2003

At 10:10 AM -0400 2003/07/03, Bruce Embrey wrote:

>  I am using Mailman 2.1.1 I have several lists and my setting for
>  SMTP_MAX_RCPTS = 500. If I have a list with 4000 addresses will
>  this setting segment the message into 8 messages with 500 recipients
>  per message? This SMTP_MAX_RCPTS = 500 was the default setting when
>  I installed it. Is it the recommended setting or should it be changed?

	Keep in mind that the RFCs only require that you allow at least 
100 recipients, and many sites lower this number even further.  Check 
to make sure that your own MTA is configured to allow as least as 
many recipients as you want to specify.

	In addition, some sites are sneaky and figure that there's a 
higher probability that you're sending spam if you have too many 
recipients per message.

	So, if you have any recipient domains that have large numbers of 
subscribers, the MTA will have to do more work to split the envelopes 
on those messages -- thus increasing the risk of multiple deliveries 
of the same message to certain sets of recipients, increasing the 
risk that some recipients won't get the message at all, etc....

	Another factor to consider is that some people set up local 
reflectors, or have their e-mail forwarded from one account to 
another.  Unfortunately, some mail systems munge forwarded mail 
sufficiently that you may get bounces from an address you can't find 
anywhere on any of your mailing lists, and you may not be able to 
figure out how to unsubscribe this user.

	Mailman supports a feature called "VERP" (which also has to be 
supported by your MTA) that allows you to have 100% certainty of 
which message was sent to which person (so long as the reply address 
isn't also munged), which greatly increases your probability of 
successfully identifying a bouncing address and removing it.

	Unfortunately, this means you send out one message for each and 
every recipient on your list.  It will take a longer to get the mail 
out, but you'll have a much higher probability of being able to 
cleanly manage your lists.

	Back in 2000, there was a fairly long thread in 
mailman-developers about large lists.  Start with 
and work your way through.

	There's also an interesting message from Chuq to 
mailman-developers that is more directly related to this issue at 
In short, he says that you probably want to set SMTP_MAX_RCPTS to be 
at least 2, and beyond 5 it doesn't seem to buy a whole lot.

	These are all issues you need to balance when setting this 
variable in your mailing list and MTA.

Brad Knowles, <brad at stop.mail-abuse.org>

"They that can give up essential liberty to obtain a little temporary
safety deserve neither liberty nor safety."
     -Benjamin Franklin, Historical Review of Pennsylvania.

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