SpamAssassin Rule: RCVD_NUMERIC_HELO
Standard description: Received: contains an IP address used for HELO
In an SMTP session the argument to the first command a client sends (EHLO or HELO) should be the client identifier. RFC2821 specifies the fully-qualified domain name of the SMTP client, the older standard RFC821 refers to the host name.
In situations that the client does not have a meaningful domain name the client SHOULD send an address literal (an IP address enclosed by square brackets).
If a Received: header is parsed that indicates the EHLO/HELO identifier was given as an IP address, but not enclosed by square brackets, this is in violation of the RFC.
The default scores for this rule can be found in the online list of tests.