If the Internet is to work successfully, it must have common policies (or standards or protocols) accepted by every company (including Microsoft, Ebay, Paypal etc.) and by every person creating software for the Internet.
Our mail servers reject all incoming emails which do not have a Message-ID: header. No exceptions.
One of the rules (or standards, or protocols or policies) is:-
3.6.4. Identification fields (on page 22)
Though optional, every message SHOULD have a "Message-ID:" field.
'Optional' and 'Should' seem incompatible. We regard 'Should' as meaning 'Must'.
Socket Labs is another of the many American companies offering
Our trusted, cloud-based SMTP relay service provides easy, reliable delivery for marketing and transactional email applications.
For example, on 2013-05-24 Socket Labs using a From: address of
sent from a Socket Labs server relay28.email-od.com [22.214.171.124] an email without a Message-ID: header. Our servers rejected the email as spam.
The email-od.com domain name is leased by Socket Labs.
World Pay is not as popular as Paypal. It's web site proclaims:-
WorldPay offers market-leading face-to-face and online payment solutions to small and medium sized business in the United Kingdom, the United States as well as online payments and fraud protection to corporate businesses globally.
World Pay's renown computer progessionals ignore error messages.
They sent emails without a Message-ID: header From: firstname.lastname@example.org
On 2013-06-11 : mm2imsmr1p.rbsworldpay.com [126.96.36.199]
On 2013-06-18 : mg4imsmr2p.rbsworldpay.com [188.8.131.52]
On 2013-07-01 : mg4imsmr2p.rbsworldpay.com [184.108.40.206]
On 2013-07-16 : mg4imsmr1p.rbsworldpay.com [220.127.116.11]
Because World Pay were too exhausted or indifferent to include a Message-ID: header, our servers politely rejected the World Pay emails as spam.