SpamAssassin Rule: UNPARSEABLE_RELAY
Standard description: Informational: message has unparseable relay lines
The Received: lines from the email are analyzed to determine the relay path. This rule matches mail that contains one or more Received: lines that cannot be parsed to extract this information.
Note that this is an "informational" rule -- in other words, it is not intended to differentiate spam from nonspam, and should not have a significant score.
The default scores for this rule can be found in the online list of tests.
The format of Received: header lines is detailed in RFC 5321 § 4.4