I don't think either are. The payment processor should be sending it, but, at least according to the RFC, it is incorrect to reject an email that doesn't have it. I suspect the reason it is SHOULD, and not MUST is for backwards compatibility with software that predates the RFC that adds the message-id header.
Maybe there is a correlation between missing that header and being spam, but then it should go to the spam folder, not be outright rejected.
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The experience with support is also similar to experiences I've had with support at many companies. I provide enough details that an engineer could probably easily fix the problem, but the support representative just dismisses it, and it is doubtful an engineer even hears about it.
I don't think either are. The payment processor should be sending it, but, at least according to the RFC, it is incorrect to reject an email that doesn't have it. I suspect the reason it is SHOULD, and not MUST is for backwards compatibility with software that predates the RFC that adds the message-id header.
Maybe there is a correlation between missing that header and being spam, but then it should go to the spam folder, not be outright rejected.
----------------------------
The experience with support is also similar to experiences I've had with support at many companies. I provide enough details that an engineer could probably easily fix the problem, but the support representative just dismisses it, and it is doubtful an engineer even hears about it.