mail -r option problem

Dear list,


I tried to use the -r option with the mail program, but whenever I use
the option I get a error like this:


odin$ mail -r abcd -s abcd rudolf.sykora@cvut.cz
a
EOT
odin$ sendmail: command failed: 550 Invalid recipient: <rudolf.sykora@cvut.cz>


odin$ mail -s abcd rudolf.sykora@cvut.cz
sdas
EOT
odin$


can anybody help me understand why?


Thanks
Ruda