Postfix: How to accept email from a specific non-existing domain?

In /etc/postfix/main.cf, place the server's IP address in mynetworks.

Example (where the IP addres is 198.51.100.43):

mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128 198.51.100.43

Right now, probably:

~ $ sudo postconf smtpd_sender_restrictions
smtpd_sender_restrictions = reject_unknown_sender_domain

Should be?

~ $ sudo postconf smtpd_sender_restrictions
smtpd_sender_restrictions = check_sender_access reject_unknown_sender_domain

option set in main.cf
You'll need to postalias /etc/postfix/access if you haven't already.

Also, chastise the sender for not owning the domain they're sending from (or not having had created the appropriate subdomain).