Much like on macOS, emails can be easily exported as EML files by dragging them from Mail.app to Files.app in Split View.
As iPhones lack Split View support, a workaround is required to export EML files:
Tap the Compose icon.
Swipe down from the top bar of the New Message window to minimize it.
Tap and drag desired email message onto the small New Message window at the bottom of the screen.
Lift your finger when the New Message window reopens; this will attach the selected email as an EML file.
/misc | Dec 01, 2023