When I think about the default setup this would make more sense:

\setupletterstyle[backaddress=no]
\setupletterstyle[backaddress][alternative=auto]

WHat do you think?

I think it would be more intuitive that way.

One more question: it would be nice if the rule under the back-address would have the same length an the back-addres. Is there a convenient way? I tried something with \textbackgrounds (example below) but without success.

\setupletterstyle
 [backaddress]
 [frame]
 [width=fit]

Thank you! I tried to set width=fit, but without the frame-option … Now one have to care of the width (I think there is no maxwidth-option for frame setups?),

so I tried to set the backgroundaddress-style to style={\ifdim\framedwidth<85mm \tfx \else \tfxx \fi}. But the problem is, that \framedwidth measures with the default font size … Or do you know a solution? ;)