Since the migration of many email accounts to the new server, we’ve had reports of email from some regular correspondents (with email hosted outside of NinerNet) to domains hosted on the new server bouncing back to those senders as undeliverable. All of these reports, so far, are about the same improper configuration of Microsoft Exchange mail servers.
A person sending you an email through a mis-configured mail server will receive a bounce message that includes an explanation for the bounce that looks like this:
you@yourdomain.com
nc027.ninernet.net #554 5.7.1 <senderdomain.local>: Helo command rejected: Go away, bad guy (.local).
The problem is the “senderdomain.local” string. In this case “senderdomain” stands in for an actual name — e.g., something that looks like it might be a domain — followed by “.local”. A properly configured mail server that connects to the public Internet is supposed to advertise a “fully-qualified domain name” (FQDN) through the “HELO” (or “EHLO”) command rather than “something.local”, which is not a real domain. Many mail servers, including ours, reject attempts to deliver mail from improperly configured mail servers advertising a “domain” that does not (or cannot) exist. The reason for this is that much spam comes from machines that are improperly configured in this manner. More technical details about this can be read in the Best Practises for Email and Network Operators – Valid HELO domain article.
Your correspondents will likely think that we are blocking their domain specifically (very likely that we are NOT) or that something is otherwise wrong on our mail server. However, it is the other way around; your correspondents experiencing this problem need to talk to their own IT people, perhaps pointing them to this post, as their mail server needs to be reconfigured correctly.
The article Exchange DNS Configuration for Email Delivery includes a number of helpful hints for the Exchange server administrator about how to properly configure an Exchange server to work correctly on the Internet with respect to domains and DNS. About half way down the page are sections entitled SMTP Banner – Exchange 2003 and SMTP Banner – Exchange 2007 that explain how to set the SMTP banner — i.e., the domain that is advertised by the Exchange server when it connects to another mail server to attempt to deliver email. As mentioned previously, this needs to be a proper domain that is resolvable on the Internet, not something that doesn’t exist like “senderdomain.local”.
Our experience is that when an Exchange server is correctly reconfigured, email from that server starts getting through again immediately, and deliveries to other servers that do not block based on this incorrect behaviour are not affected.
Another possible solution to this problem is for the Exchange server to use a smart host, through which all outbound email is delivered to the public Internet. This has a number of advantages, including not having to reconfigure the SMTP banner and the fact that the server administrator doesn’t have to be concerned about their own IP address being added to a block list if (again as a result of mis-configuration) the server inadvertently becomes the source of spam. NinerNet provides this service (relay server / smart host) for USD30 / CAD36 / ZMW165 per month.
Or you could send Microsoft Exchange Server 2007 For Dummies to the sending domain’s server administrator!
Update, 2022-01-24: The information above applies to any domain or sub-domain used in a mail server’s HELO command, not just the specific nonsense sub-domain “senderdomain.local”. If the maintainer of the sending mail server makes up a sub-domain like “mailserver.mydomain.com”, but doesn’t actually create an A record for “mailserver.mydomain.com”, then the effect will be the same, their email will not get through.
Additionally, these days the error message is different. It is as follows:
450 4.7.1 <mailserver.mydomain.com>: Helo command rejected: Host not found
Mail server admins are still making this mistake today, in 2022!
