Where are sent emails stored on the device?

I have encountered a strange thing. I was composing an email but half way through I realized i had to refer to something in an earlier email from the same person and the only way to get back to it was to exit the email I was composing which put it in Drafts. When I got back to finish my draft and send it, it bounced with the recipient's server complaining that my email was "not RFC compliant" because it contained "multiple From headers".

I can successfully reproduce the issue at will, as long as the recipient's email is blablabla@gmail.com. I have tried other destinations but not yet found one that triggers the same issue.

When I inspected my sent email, the one that gmail was complaining about, in another email client that can show the full headers, I do not see any duplicate headers, but it might be that my client is trying to be clever and merging duplicates.

Hence my question, where are the sent emails stored on the device, in their raw form? The obvious places did not yield anything useful.