$FAILED_TESTS includes full stdout/stderr of tests and stack traces. If you are using something like
these can be quite large. Furthermore, sometimes complete text of a log session is packed into a single line of text (no internal newlines), and when replying to such a message, Thunderbird has extremely poor performance.
Better to truncate all this to some reasonable maximum sizes, as we do for e.g. console logs; the user can always click the hyperlink to the build's test cases to see details.