I know this is an old thread, but I wanted to add a little more texture to the answers.
There is, in fact, a problem with the sender's domain. As @milli pointed out the PTR and A records should match - and they do not.
if postfix is configured with reject_unknown_sender_domain, postfix performs the following checks...
1) retrieve PTR record from dns - if there is no PTR record, reject the email
2) retrieve A record from dns for the domain in the PTR - if the IP in the A record does not match the IP of the connected MX, reject the email.
For instance, an MX connects from the IP 184.108.40.206
$ host 220.127.116.11
18.104.22.168.in-addr.arpa domain name pointer wj-in-f27.1e100.net.
$ dig a wj-in-f27.1e100.net. +short
In this case, the A record from the domain in the PTR record contains the same IP address as the host that is connected. This email will NOT be rejected.
The "problem" in most cases, though, is illustrated in the following:
$ host 22.214.171.124
126.96.36.199.in-addr.arpa domain name pointer jalequity.com.
$ dig a jalequity.com. +short
The A record from the domain in the PTR record doesn't match that of the connected host - this email will be REJECTED with "Client host rejected: cannot find your hostname"
The most common mistake that leads to this is putting the actual domain (in this example "jalequity.com") in the PTR record, rather than a subdomain. To fix this, the sender should amend their PTR record to something like mail.jalequity.com, then add an A record for mail.jalequity.com with the IP address 188.8.131.52. Any further email sent from that server would be accepted.
The BEST fix is for the sender to correct their DNS records.
The ALTERNATE fix is to loosen your postfix restrictions from "reject_unknown_sender_domain" to "reject_unknown_reverse_client_hostname". But there are consequences to this action - primarily that most spammers have a PTR record (it just doesn't match an A record), so relaxing the postfix restrictions will ensure that you don't miss an important email, but it will also ensure that your spam filters have constant work!