Specifying <forward-path>??

["M.C. Vernon" <mcv21@cus.cam.ac.uk>]

Can anyone tell me how to specify the exact forward path of mail - 

the rfc says that 

@host1,@host2:user@host3 is valid, but how can I tell sendmail (or any
other mail sender) to do this - the idea is to use this in a programme -
so it will generate the file and then | it to mail...

Thanks,

Matthew

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