Plain text still needs structure. Line breaks, quote markers and whitespace tell a reader which words belong together and who wrote them. A message may look clear in one window and confusing in another.
Hard breaks and display wrapping
A hard line break is part of the text. Display wrapping is a choice made by the reader's application. Repeatedly inserting hard breaks to fit a particular window can make a paragraph awkward when the same message is displayed at a different width.
> A short quoted statement. A response to that statement. A separate point starts a new paragraph.
Keep quotations identifiable
Use an unambiguous quote marker and retain the part needed for context. A reply inserted without a visible boundary can be mistaken for the original author's words. Rewrapping text should not silently remove or move quote markers.
A small interoperability check
Prepare an ordinary paragraph, a short quotation and a line whose spaces matter. View the message at two widths, then reply to it. Check whether the quotation remains clear and the meaningful whitespace survives. This is a portable test method, not a claim about the behavior of any current SeaMonkey release.