The "love note" I sent:

When the detailed view of headers is turned on, the Yahoo Domain Key thing has an excessively long unbroken string of characters, making my page display up to twice the width of my screen, making me scroll from side to side.

This is not cool.

I hate to think that *my* e-mails are doing this to other people. If they are viewing full headers, they probably are.

I had an inexplicable problem some years ago with messages from a Yahoo Group displaying wider than my screen, even though the message content was not exceptionally wide. I do believe I've found the culprit.

Proposed technical fixes:

a) Admit Domain Key is stupid and no one else uses it and stop using it yourselves
b) Force breaks in the Domain Key after every 80-160 characters (which will stop it from causing side-scrolling in anybody else's webmail)
c) Have a toggle section per message to hide full headers in addition to a control panel setting
d) Force your header display to go to the next line after so many characters, even if there is not a natural breaking character
e) (my favorite) both b and d.

Though actually, since I have set my settings to break my lines after 80 characters or somesuch, while the headers may be wide for messages I send, at least my text does not wrap to the length of the header as with HTML-formatted e-mails. So I probably don't break people's pages, for those who use webmail and display all headers and have a stupid web client like fucking Yahoo here.