Do not look in logs for the senders domain name, it is probably rejected before the domain name had a opportunity to appear.
Launch a tail -f on the mail.log and send the test email.
tail -f /var/log/mail.log
Look for any connections that are rejected and for a reasons. The log entry does not have to include the senders domain name, because the sender's smtp server was probably rejected before it had a chance to send "mail from:" command. It could be dropped because of ehlo header, ip, or other rule. It must be there somewhere in the log.
Does the sender's domain has proper dns configuration? mx? any openspf entries? Does the sender's smtp has reverese dns entry?
If there is no connection initiated in the logs, that means the sender's server didn't even try to send the mail. Does your domain has proper dns configuration? Are the MX entries valid?
dig domainname mx
dig domainname txt
Or maybe the sender's domain is configured as local destination in your postfix?
You did not provide any real domain names, so we can't check anything, and it is really impossible to guess. There is a lot, a lot, of reasons for not receiving emails.
The dns entries for both domains look correct. Look for ip of the senders domain smtp server (184.108.40.206) in your /var/log/mail.log