Stainless Steel Rat <ratinox@peorth.gweep.net> writes:
> there is no requirement that a Message-ID string contain a domain name.

Yes, there is.  RFC822, page 44, appendix D, syntax rule listing:

     optional-field =
		 /  "Message-ID"        ":"   msg-id
     msg-id      =  "<" addr-spec ">"            ; Unique message id
     addr-spec   =  local-part "@" domain        ; global address
     domain      =  sub-domain *("." sub-domain)
     sub-domain  =  domain-ref / domain-literal
     domain-literal =  "[" *(dtext / quoted-pair) "]"
     domain-ref  =  atom                         ; symbolic reference
     atom        =  1*<any CHAR except specials, SPACE and CTLs>

Lest anyone think this allows OWHN (one-word hostnames) on the
right-hand side of `@' (which was OK in RFC822's pre-DNS days), note
the "global address" comment above on addr-spec -- a OWHN was
sufficiently "global", prior to DNS -- and see that RFC1123 expressly
denies this in §5.2.18 anyhow:

         Errors in formatting or parsing 822 addresses are unfortunately
         common.  This section mentions only the most common errors.  A
         User Agent MUST accept all valid RFC-822 address formats, and
         MUST NOT generate illegal address syntax.
	 ...
         o    Some systems fail to fully-qualify domain names in
              messages they generate.  The right-hand side of an "@"
              sign in a header address field MUST be a fully-qualified
              domain name.
	      ...
              DISCUSSION:
                   Although RFC-822 allows the local use of abbreviated
                   domain names within a domain, the application of
                   RFC-822 in Internet mail does not allow this.  The
                   intent is that an Internet host must not send an SMTP
                   message header containing an abbreviated domain name
                   in an address field.  This allows the address fields
                   of the header to be passed without alteration across
                   the Internet, as required in Section 5.2.6.

--karl