Keep what you already have
Do not discard screenshots, printouts or PDFs because a native message has been requested. They may establish what was presented to someone at a particular stage. Preserve the submitted files and record who supplied them, when, and how. A later screenshot taken from an edited mailbox is a different evidential item.
What native material adds
An email file may contain headers, alternative text and HTML bodies, attachment data and technical properties that are not visible in a normal reading view. MIME is the structure used to represent different message content types. A native file is still a supplied artefact whose provenance requires assessment; its extension is not a certificate of authenticity.
A practical comparison
For a disputed sentence, compare the screenshot with available sender and recipient copies and any independent records. Record differences before attempting to explain them. A display setting, export conversion or omitted quoted text can create a visual difference without proving dishonest alteration. Conversely, a convincing appearance does not establish that transmission occurred. The appropriate next step is to identify the original source and preserve the available representations separately.
Sources & further reading
Primary references support the technical explanation. They do not establish D3 validation of a particular method.