In our company we recently had some discussion about this topic.

For western languages it is widely accepted that for best 
readability the length of lines of text should be something 
between 60 and 65 characters per line.  

This means Roundup should normally store text in format=fixed (as
opposed to format=flowed as discussed in
http://www.ietf.org/rfc/rfc3676.txt ).  
However E-mails containing text with format=flowed became 
increasingly popular. Certain foreign languages (like chinese) 
may even require some other special handling. (see 
https://sourceforge.net/p/roundup/mailman/message/34879502/ )

AFAIK currently (1.5.1) the mail gateway of Roundup does nothing with 
emails containing huge lines as seen here in msg4993 and store them
as is.  To get those displayed in readable form with reasonable 
short line lengths the user is forced to narrow the width of the
browser window.

It might be an option to pipe incoming emails through the 
utility program ``fmt -w 65`` to enforce hard line breaks before
the emails gateway hands them over to Roundup for storing.