Because of lost and delayed emails during the changeover to TMC, I did tests sending test emails from my gmail.com address to TPG Post Office. I did this late evening of 15/10 and evening of 16/10. There was some inconsistency across the two durations. It looks like Post Office will not receive emails if the length is less than approx 9,600 bytes.
My test emails had a 4 byte subject and variable length message text. Subject of 4 bytes and message text of 97 bytes was the shortest that Post Office would receive. The other variable is the length of control records inserted by email servers between gmail.com web server and TPG Post Office. The length of control records can amount to at least 9,500 bytes. The greater the length of control records, the less user text is needed to make Post Office receive the email.
This is not suitable behaviour. An email with no subject and no text could be significant to a recipient. Gmail can send an email with no subject and no message to my outlook.com address, albeit with a warning to me.
Post Office will send an email with no subject and no message with no warning. So it should be able to receive the same email (no subject, no message).
