Messing with the syntax is a really bad idea IMO. I understand that
there is some discomfort in embedding information in tokens like
this, but it has the feature that it beats the alternatives.
And the feature of production code.
I might feel differently if I felt this was going to turn into a
completely open-ended thing. But I see little chance of our adding
additional orthogonal security facilities to SMTP in the future. What
you see is all there is.
If Submit warrants adding two/four more tokens (see private mail from
Chris), clearly extensions other than security facilities can add
I do hope there won't be more of these tokens, but I won't be surprised
if future i-d authors have very fertile imagination. The designers of
RMX, DMP and SPF spring to mind.