Relay access denied (454)

I have trouble with postfix sending e-mails to any destination (Relay access denied) And yes, I googled a lot, tried a bunch of things but with no luck. Here is the thing: I am trying to use Mailman3 as a listserver with Postfix to deliver e-mails to any destination. If I am requesting access to a list, normally an e-mail is sent to the given e-mail-address. This fails with a 454-error.

NOQUEUE: reject: RCPT from localhost[::1]: 454 4.7.1 <[email protected]>: Relay access denied; from=<[email protected]> to=<[email protected]> proto=ESMTP helo=<listserver.domain.com>

Here is my main.cf. Stripped down..

alias_database = hash:/etc/aliases
alias_maps = hash:/etc/aliases
append_dot_mydomain = no
biff = no
compatibility_level = 2
default_destination_concurrency_limit = 15
default_destination_recipient_limit = 30
header_checks = regexp:/etc/postfix/header_checks
inet_interfaces = all
inet_protocols = all
local_recipient_maps = hash:/var/lib/mailman3/data/postfix_lmtp
mydestination = $myhostname, localhost.$mydomain, localhost.localdomain
myhostname = listserver.domain.com
mynetworks = 127.0.0.0/8
recipient_delimiter = +
smtpd_banner = $myhostname ESMTP $mail_name (Debian/GNU)
transport_maps = hash:/var/lib/mailman3/data/postfix_domains
unknown_local_recipient_reject_code = 550

I don't know what's going on. I don't get it. Can someone please enlighten me?

Thank you very much.

It is not totally clear from your question which domain you want to accept emails for. Based on the log you provided I assume it's domain.com.

Have you checked /var/lib/mailman3/data/postfix_domains file content? Do you have entries for domain.com in it?

(The file won't be generated until you added the domain and perhaps a mailing list in Mailman.)