Greetings. On a server running Postfix and Mailman 3.3.9, the Postfix queue shows 20-30 entries/day like this mildly obfuscated example:
4Y8M1n5v5VzHSpQ 2023 Thu Dec 12 10:48:45 postmaster@example.com (host gmail-smtp-in.l.google.com[142.250.101.26] said: 450-4.2.1 The user you are trying to contact is receiving mail at a rate that 450-4.2.1 prevents additional messages from being delivered. Please resend your 450-4.2.1 message at a later time. If the user is able to receive mail at that 450-4.2.1 time, your message will be delivered. For more information, go to 450 4.2.1 https://support.google.com/mail/?p=ReceivingRate 41be03b00d2f7-7fd409820c4si13081249a12.439 - gsmtp (in reply to RCPT TO command)) baduser2000@gmail.com
From the Mailman logs it appears these are responses to signup attempts to a publicly advertised list hosted on this server. The signups come from many different Gmail addresses.
I'm concerned about being part of a botnet. Other than taking the list private, is there some other means I can use to block/throttle these signup attempts?
Thanks
dn