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Asked: 2026-05-10T15:14:44+00:00

In short: I’m trying to figure out if I should tell a mail administrator

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In short: I’m trying to figure out if I should tell a mail administrator of a friend’s employer whether their mail configuration should be fixed, or if I should revise my own policy to be more liberal in what I accept, or neither.

A friend was complaining of being unable to reach anything on my mailserver. I dug into it and it seems that the hostname provided by his mail server when it connected to mine was somewhere in the *.local space, meaning it wasn’t globally resolvable.

They were rejected with ‘Helo command rejected: Host not found;’ by my postfix mailserver. I’m perhaps strict on my UCE checks in postfix, so I whitelisted their (in my opinion, misconfigured) server but now I’m trying to figure out to what extent they actually are misconfigured, versus whether I’m just being too harsh in what I accept.

So then I checked the RFCs – RFC 821 says ‘The HELO receiver MAY verify that the HELO parameter really corresponds to the IP address of the sender. However, the receiver MUST NOT refuse to accept a message, even if the sender’s HELO command fails verification.’ which suggests to me that I’m actually the one violating the RFC.

Was this portion of RFC 821 ever replaced by a future RFC, that I can point to? Or must mail servers accept mail with bogus HELOs? Are there any well respected authorities I can point to that state the HELO hostname should be valid, as a reference for contacting their mail admin?

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  1. SMTP RFCs do not require it, but lots of popular systems will reject mail with bogus HELOs. Note that RFC 1033 and RFC 1912 both require all internet-reachable hosts to have a valid name; simply listing that name in the HELO will fix many problems. Some spam filters, unfortunately, also reject mail from hostnames containing strings that imply they are in dynamic address pools (e.g. ‘dynamic’, ‘dsl’, or a dash-separated IP address as is common with many ISPs).

    One option if your friend does not have control over their reverse DNS is to use a suitable machine as smarthost for outgoing mail; e.g. their ISP’s mailserver.

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