Updated: 2024/07/12

Postfix main.cf:
smtpd_sender_restrictions =
 texthash:/etc/postfix/sender_access

Create /etc/postfix/sender_access containing rejected senders:
# Entire domain domain.com
domain.com REJECT
# All sub-domains of domain.com
.domain.com REJECT
# Individual user
person@domain.com REJECT

Set proper permissions:
chmod 644 /etc/postfix/sender_access


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