| The ABNF has never allowed spaces between the header "tag" (for want of
| a word) and the ":" (going back to RFC 822).
No, that's incorrect (or misleading anyway). While if you just read 822's
ABNF there are no spaces allowed, there aren't any anywhere else either.
The ABNF needs to be read in conjunction with the text, which in 822 permitted
white space (including comments) between any two tokens. That includes
the field name and the following ':', so in 822 it was perfectly legal to do...
Of course Robert is correct, but RFC 5322 makes this more explicit (see
§4.5 and the ABNF therein). I did not really understand this until last
night, however :-) RFC 5322 says that the obsolete syntax is a MUST to
interpret, which is unfortunate. Although I suspect that if you sent out
an email today with spaces between the header and the colon, a number of
MUAs would choke.
--Ken
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