+1 I have experienced the same issue and it’s pretty major.
Basically, what I am learning is that discourse forums are just completely different beasts from mailing lists. It appears that we need to re-educate ourselves and our communities if moving to discourse from mailing lists. The best way to do this may be to force ourselves to log into discourse website and create a new topic (or multiple topics!) from the email, quoting accordingly. Being disciplined about doing that will benefit everyone ultimately because it will be easier for others to respond and engage with what you are contributing (rather than slogging through one long string of emails that have been forwarded into a forum).
But still it does happen sometimes that we are in a hurry and just want to share an email with our team no matter what it looks like, with some context and instructions at the top. Perhaps the answer would be a simple code in the email eg
[[[[[ to tell discourse not to stop parsing when it hits a signature or forwarded text etc. Or how about having the import script store both the parsed/filtered version for the forum and as an attachment the full text of the email in case something seems to be missing or was not captured?
Here’s some interesting code on github from og_mailinglist (a Drupal module turning organic groups into mailing lists) to filter out the ugly stuff - it actually keeps it (though replaces email addresses with