Message unknown command: "xsender"

[Megias Sanchez, Jose Manuel]

        Hello I've installed qpopper version 4.0.3 in a PC running FreeBSD
4.3 and sometimes it appears the following message:

mihost# Jun 29 10:31:25 mihost qpopper[82671]: jms2 at jms.mi-dominio.com
(10.130.105.3): -ERR Unknown command: "xsender".

the mail is working fine but what does it mean this message?. Thanks in
advance.