This document is intended for those operating their own mail servers or smarthosts and/or systems or software that send mail through automated or scripted means. Examples of this would be an office PBX system sending voice mails to its users via email or a crontab on a remote server sending a report to an email address where the email accounts are hosted on NanoServerss mail servers.
This error can happen for a few reasons. It is usually generated as a result of a restriction placed on the mail servers to avoid receiving improperly formatted (and possibly infected or spam) mail.
One reason for this, although unlikely, is because of the hostname your server is introducing itself has when sending the "HELO" or "EHLO" command to our mail server is incorrect. This problem may occur if the sending server is not in accordance with the RFC 821 and RFC 1869 documents. The receiving mail server expects either a host name only or a fully qualified domain name (FQDN) following the EHLO/HELO command. A trailing dot after the FQDN is interpreted as incorrect syntax, and a 5xx error message is generated. If the fix below does not provide a solution to this problem Im afraid you may have to contact the vendor of the PBX/Voicemail system for information.
The other reason for this, which is more common, is the restriction that an email must have a valid "FROM" address when setting up the session with our server. Without a valid "FROM" address the email is not RFC compliant and most mail servers will not accept mail under these circumstances these days (some will, but most wont). This restriction is growing in popularity to avoid email being sent to and through mail servers that cannot be bounced.
If the email is being sent from software that you cannot control the FROM address you must contact the vendor. Otherwise, please check the settings of the software in question and ensure the FROM address is set.
