I believe iCloud either sends an empty plaintext part or otherwise sends a multipart message with an empty first part. However, I am not going to investigate further because this feature is going to be removed.
Because of EU's GDPR and starting May 25th it will not be allowed to accept personally identifiable information, such as those provided in support tickets, via email from EU citizens. If a business does receive such an email they are to warn the client that what he's doing is insecure, offer them a secure alternative and delete the email, basically pretending they never received it. That makes the whole email-to-ticket feature impossible for EU citizens. The only workaround to that would have been support for encrypted emails but as far as I can see adding PGP or S/MIME support in ATS would be nothing sort of a mighty task - let alone the fact that the clients would have to learn how to send encrypted emails.
Between the technical and legal issues surrounding GDPR and email to ticket functionality my decision is to remove this feature starting with version 3.
Nicholas K. Dionysopoulos
Lead Developer and Director