SMTP 551 code in use?

Hi,

when I (acting as SMTP client) send rcpt to and receive a 551 code from 
a server, it might be a good idea to use the referenced e-mail address 
instread of the one I used. Final decision left to the user, of course.
However, I can't find any formal grammar for the text supplied by 551. 
Is there one? Can I expect server implementations to conform to this 
well-hidden grammar (assuming there are server implementations that 
return 551 at all)?
--Arnt