The default amavisd.conf configuration contains a @blacklist_sender_maps configuration that matches some special senders such as “optin@something” and directly blocks the email regardless of score.Good catch with optin. It seems to be [1]: 2.9 + 5.0 > 6.31 ($sa_kill_level_deflt).
[1] https://gitlab.com/amavis/amavis/-/blob/v2.13.1/conf/amavisd.conf?ref_type=tags#L282
