From a colleague:

Your mail client doesn’t play nicely with mine (Mac Mail).
As you can see by the screen snap, it doesn’t wrap text and includes the 
long version of headers on forwarded messages.
As a result, I sometimes don’t read stuff from you because it’s too 
annoying to scroll and scroll…

End quote.

A web search does not reveal anything about this issue.  I do not have 
an Apple MacOS X box; all Linux and MS Win Thunderbird users do not 
report this issue. I am using IMAP for reading email, and an external 
SMTP server for sending email.  Is there a (SL) Thunderbird 
configuration that is more Mac Mail friendly?

Yasha Karant