I'm not sure if this is a bug in x-remote or not, but it starts there. If you use x-remote to open a mailto: url, the mailto: ends up in the compose window. I'm not sure why but people have been mucking around in that code recently. Might have been caused there.
worksforme with linux trunk build 20020608
WFM Linux Moz 1.1.alpha (build 2002061108) You can look at the blame log and line 237 to see when blizzard made some fixes related to this: http://bonsai.mozilla.org/cvsblame.cgi?file=mozilla/xpfe/components/xremote/src/XRemoteService.cpp#237
Marking WFM due to the last comment. Reopen if your opinion is different and you can reproduce this with a current build.