Linux Harbour

Sometimes we received enquiries from our client when one of their business partners/customers could not send emails to them successfully. And we found that the problem is normally caused from opposite Windows email system, which is not configurated well in internet RFC document.

The following line of email reject information is found in email log file.

May 28 17:03:47 mail postfix/smtpd[30547]: NOQUEUE: reject: RCPT from smtp.xxxxxx.com[xxx.xxx.xxx.xxx]: 504 swhkg0111: Helo command rejected: need fully-qualified hostname; from=xxxxxx@xxxxxx.com to=someone@our_client.com proto=ESMTP helo=swhkg0111

It means when this poorly configurated Windows email system (or spammer sometimes) says "Hello" to our client's email system with their non-full-qualified hostname, so the email system is rejected from sending emails to us.

This case is caused by one of UCE options (reject_non_fqdn_hostname) in postfix, it is explained in Postfix document regards to UCE.

reject_non_fqdn_hostname

Reject the request when the hostname in the client HELO (EHLO) command is not in fully-qualified domain form, as required by the RFC. The non_fqdn_reject_code specifies the response code to rejected requests (default: 504). We hopes these system administrators should work on email system administration in normal practice.

Reference:

• RFC 2821 - Simple Mail Transfer Protocol by The Internet Engineering Task Force (IETF) of The Internet Society