Email Error: 450 Client host rejected

A couple of our clients sometimes have issues when sending email, with a returned non-delivery report stating the following:

Peer server rejected email:

450 Client host rejected: ‘cannot find your hostname’

It turns out this is a very strict check (usually performed by postfix), that is controlled via the directive reject_unknown_client_hostname in the postfix configuration. The documentation for the directuve can be found here:

http://www.postfix.org/postconf.5.html#reject_unknown_client_hostname

As per the link above, the 450 error is returned when:

1) the client IP address to name mapping fails

-or-

2) the name to address mapping fails

-or-

3) the name to address mapping does not match the client IP address. 

the solutions to each of the issues above are all related to the DNS infrastructure:

1) Ensure you have the correct PTR (reverse record) that returns a valid hostname for your outgoing email server’s IP address. Example:

host 78.133.115.83
83.115.133.78.in-addr.arpa domain name pointer compunet.com.mt.

2) Ensure that the hostname returned in the SMTP greeting and that returned in step (1) both resolve back to the correct IP address. Example:

host compunet.com.mt
compunet.com.mt has address 78.133.115.83

3) Ensure that your public IP for the email server matches that returned in (2)