On 2022-08-23 14:31:55 +0100, Martin Gregorie wrote: > Fair enough: I did say that some of this was off the top pf my head at > the end of a longish day. > > Would doing the lookup trick on the URL in the Message-ID header be any > more reliable?
DNS Lookup checking is valid only for IP -> FQDN -> IP, not FQDN -> IP -> FQDN, whatever the FQDN. Note also that the Message-ID is not a URL, just some kind of identifier with some syntax and that must be unique. In general, you shouldn't assume anything on the right-hand side of a Message-ID. RFC 5322 just says: As with addr-spec, a liberal syntax is given for the right-hand side of the "@" in a msg-id. However, later in this section, the use of a domain for the right-hand side of the "@" is RECOMMENDED. In particular, it doesn't need to be resolvable, and there are good reasons for which this may not be the case. For instance, the Message-ID may be generated on the machine from which the message is sent, with some internal hostname on the right-hand side (this is better to ensure unicity), thus is not resolvable. -- Vincent Lefèvre <vinc...@vinc17.net> - Web: <https://www.vinc17.net/> 100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/> Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)
