Two things to check, bearing in mind that:
- The clipboard generally contains, at any given moment, several different pboarditems, each representing the current data in a different format, and each with its own identifying label.
- the choice of which of these pboarditems to use, when a paste is requested, is up to the receiving application – in this case Mail.app
First thing to check, is Mail in plain text or rich text mode, when this happens ?
Second, what set of pboarditems is actually in the clipboard when this happens ?
(There’s a Clipboard Viewer macro on this forum, which shows the bundle ids of the pboarditems currently in the general pasteboard)