An SMTP HELO/EHLO is used within the SMTP transaction to identify the sending mail server. It is BCP (best common practice) for any ISP or organization operating a mail server on the Internet (and in fact defined by RFC 2821, page 29, section 4.1.1.1, read in conjunction with the definition of "Domains" outlined in section 3.6, page 22 and section 4.1.2, page 37) that when an SMTP transaction is begun, the client (sending mail server) is to identify itself using HELO/EHLO followed by a fully qualified domain (hostname) or address literal. A domain is defined as being a combination of letters or digits, followed by a dot "." and a combination of more letters or digits. For the purpose of RFC2821, it is defined that a domain is a fully qualified and resolvable name. An address literal is an IP address enclosed in square braces [x.y.z]. It is policy of AllWorldIT to reject any and all attempts to deliver mail when there is...

1. No HELO/EHLO issued; or
2. The domain name given in the HELO/EHLO is not a fully qualified domain name (FQDN); or
3. The domain name given in the HELO/EHLO does not resolve.

There is no possibility this policy will change in the foreseeable future. Whitelisting hostnames, IP space, senders or domains is not possible. If your mail server wants to make use of our network to deliver mail, it will have to adhere to BCP and the relevant Internet standards (RFC 2821) and identify itself with a fully qualified resolvable domain name.

Sending servers attempting to use a forged HELO/EHLO of any host (FQDN or address literal) on the AllWorldIT network will be automatically blacklisted for 28 days.

Errors receiving mail?
If you are having trouble receiving mail and the error returned to the sender contains "Helo command rejected: need fully-qualified hostname;" or "Invalid HELO/EHLO: reason stated here", it means the sender's mail server has not been set up correctly and does not identify itself in accordance with best common practices and Internet standards. The sender will need to contact their ISP or IT department, show them the error message and ask them to resolve the issue. There is nothing we can do to fix the sending mail server's configuration.

The folowing RFC describes in detail the SMTP protocol as well as the HELO/EHLO command...

RFC 2821, page 29, section 4.1.1.1 describes the HELO/EHLO command and states the following "The argument field contains the fully-qualified domain name of the SMTP client ...".

RFC 2821, page 22, section 3.6 defines a domain as being fully qualified and resolvable... "Only resolvable, fully-qualified, domain names (FQDNs) are permitted when domain names are used in SMTP.... Local nicknames or unqualified names MUST NOT be used... The domain name given in the EHLO command MUST BE either a primary host name (a domain name that resolves to an A RR) or, if the host has no name, an address literal as described in section 4.1.1.1...".

RFC 2821, page 37, section 4.1.2 defines the syntax for a domain name.

References:
SMTP Standard - RFC2821
MAAWG BCP (III. c. iv)

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