Okay, thanks for your answer. Strange. I'll try it on another computer. On this one it's not possible to unpack the portable version, no matter on which partition or folder.
"Mailbird 3.0.27 RePack (& Portable) by elchupacabra" can't unpack the portable version. Error message (in german) is: «Die Syntax für den Dateinamen, Verzeichnisnamen oder die Datenträgerbezeichnung ist falsch».
Link doesn't work.