erik@dante.de wrote on 2024-07-29 12:28:
status=deferred (host gmail-smtp-in.l.google.com[74.125.128.27] said: 421-4.7.28 Gmail has detected an unusual rate of unsolicited mail. To protect 421-4.7.28 our users from spam, mail has been temporarily rate limited. For 421-4.7.28 more information, go to 421-4.7.28 https://support.google.com/mail/?p=UnsolicitedRateLimitError to 421 4.7.28 review our Bulk Email Senders Guidelines.
Is there a solution with which Google delivers the mailing list messages without complaining, but the sender email address is also in the “From:”?
I've found that setting this in Postfix's main.cf file to help: it sends each message separately, eliminating the "bulk" complaint:
default_destination_recipient_limit = 1
https://www.postfix.org/postconf.5.html#default_destination_recipient_limit
The default maximal number of recipients per message delivery. This is the default limit for delivery via the lmtp(8), pipe(8), smtp(8) and virtual(8) delivery agents.
There are related tuning options that may be worth investigating.