This is a known Issue. For More Information see: Email header malformed when using SendMail
This is one of many acknowledged bugs that can be found in the Known Bug List here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: http://www.4shared.com/file/8orL8apk/FMP_Bugs.html
This is a trivial bug to fix. The only thing that needs to change is adding a domain (eg, "filemaker.com") to the right side of the "@" symbol for the message ID to conform to RFC 5322 §3.6.4:
'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
Notice that the right-hand side of the "@" (ie, "id-right") is NOT optional:
'msg-id = [CFWS] "<" id-left "@" id-right ">" [CFWS]'
Source URL: RFC 5322 § 3.6.4
While I am definitely not recommending such, the domain can even be constant for the entire FM product line (just not empty).
I still have a project that is dead in the water until this bug is fixed (and has no possible budget for a plugin).
This was an advertised feature of FM and, morally, Filemaker should fix it now.
I realized recently that there is a much more simple fix that FileMaker can make literally is seconds:Don't create any message ID at allWhile the bug should be fixed with correct message ID creation, leaving out the message ID entirely will work. It's just not OK to put in half of the message ID.
It seems that FileMaker Pro 11.0v3 now DOES just leave out the Message-ID completely. The problem is that many spam filters, including GMail, count that as a hit against a message as far as spam probability.
It seems like the best fix would be for the part after the @ to by the domain name of the FROM address. Obviously, the perfect solution isn't easy, but that seems least likely to be a problem.
Same thing in FileMaker Pro Advanced 11.0v4 - the Message-ID is left blank.
Doing more research, it seems that the intermediate mail server (for an address that forwarded to GMail) stripped OUT the invalid (nothing after @) Message-ID. When the message is sent directly to the GMail account, the invalid Message-ID described above is present.
So, it seems like Send Mail (via SMTP) is only safe for use when messages are sent only within an organization, not for general sending of email. Very disappointing.