From the gkrellm mailing list (April 2009): >When using external mail check program, gkrellm doesn't properly check >eof of the open pipe, and therefore only runs the program once at >startup. To reproduce the bug, just set the program to some popup (e.g., >kdialog) and click on the envelope (forced check). Nothing will happen. The fix is trivial and available at the given URL. It has been added to Debian's gkrellm-2.3.2 package. Unfortunately, the upstream maintainer has not merged the fix so the patch is still necessary for 2.3.4.
Good news: This is fixed in 2.3.5, which is now in portage.
