The mailing list for listmasters using Sympa

[Steve Shipway <address@concealed>]

On Tue, 2020-04-28 at 15:49 -0700, Matt Taggart wrote:

> Hi sympa-users!
>
> We run a large sympa server that can receive email from the internet.
> Lately we have seen an interesting problem:
>
> * have a list that's configured with "send" to only allow member
>      posting, and other messages are rejected (or similar send scenari)
> * spammer uses a 3rd party SMTP server to send mail to the list address
>      with a forged From address, which is not subscribed to the list
> * sympa rejects the message and sends a bounce to the forged address
> * the forged address is a spam trap and then our mailserver gets listed
>      on an RBL and we get listed and have to argue with the RBL about why
>      we sent mail to a spam trap

You could create a custom scenario for posting, that examines the message header, and silently rejects messages for unsubscribed users that have certain headers.

For example, if your mail gateway already performs spam scans and adds headers for SPF, DKIM, and spam score, you could make the reject silent if there is a failed SPF/DKIM, or no SPF check, or spam score is over a certain amount.

Also, depending on the nature of the list, you could ask all posters to have an SMIME certificate and silently drop anything not SMIME signed.

As long as your 'posting refused' message contains all the standard things (DKIM signed, SPF passes, links to mailing list and admin included, etc) then whoever owns the spamtrap should notice that this is neither spam nor blowback, but some don't always take the time to check.

Steve

-- 

Steve Shipway | Senior Email Systems Administrator 
Phone: +64 9 302 0515 Fax: +64 9 302 0518 
Freephone: 0800 SMX SMX (769 769) 
SMX Limited: Level 10, 19 Victoria Street West, Auckland, New Zealand 
Web: http://smxemail.com