/home/vpopmail/bin/clearopensmtp tells me that vpopmail was not compiled for roaming users. In cron I've redirected the output to /dev/null so I was not able to see this error.
I've removed the directory source of vpopmail and recompiled and now it works. May be that a "make clean" in the vpopmail directory source before recompiling could help in the future? :)
