Schulz via Mailman-users wrote on 2024-08-24 12:39:
if this particular subscriber uses apple mail on iphone, and the mails in questions are_answers_ to existing mails, it might be the known issue that the References: header grows beyond a limit given by exim. This is the fault of apple mail and violates standards, historically also microsoft didn't care about that limit. I haven't found a workaround yet; postfix mailer allows to split this line.
This sounds very plausible.
Another thought, less likely but worth looking into, would be in Postfix's main.cf:
mailman_destination_recipient_limit = 1 ## default_destination_recipient_limit = 1
As I understand it, Postfix batches outgoing messages to reduce overhead, so perhaps one member in a batch is failing and Mailman is applying the bounce to everyone in that batch?
Forcing Postfix to send each message individually might help and is simple to implement.
rb