When I'm trying to paste a clipboard contents being writing a new mail message nothing happens.

Sure thing, Emacs is related to this issue because I've tried to paste a selection from an Emacs buffer. But it wouldn't be right to blame Emacs and only Emacs for that, because I've never face this problem before --- pasting the same contents works fine in many (all?) other software I use.

A little research I've made shows, that there are some strange type tags that Emacs annotates the clipboard data with. And the reasonable way to work with them is just to ignore them: i.e. to ask the clipboard for special data only when its tag really looks like a MIME type.

There is the patch for qlipper that manages with that kind of problem: https://github.com/ufleisch/qlipper/commit/f60ac59184149b468d21b1bb5d27535a75e03d7f

With that patch, I've made another one --- for Claws Mail: http://git.altlinux.org/people/manowar/packages/?p=claws-mail.git;a=commitdiff;h=ffd34a810898daf59298af5707e9d129e87aae0b .

I don't thing there will be any regressions with this patch, so I'm asking you to apply it.