You've managed to miss a few of the biggies I'd count:
- the charset parameter is nowhere near as correct as you'd like it to be
- many RFCs are not followed in practice. In particular, RFC 2047 is literally more often incorrectly implemented than correctly implemented.
I also once recall trying an admittedly weird message (a message whose body is of type message/rfc822) on 4 different IMAP server implementations and getting 4 different answers for part numbering.
I'd say building an email client is harder than building a web browser because at least the people who write the specifications for the latter have been trying to thoroughly document all the idiosyncrasies and how to handle them.
Oh God, IMAP part numbering. I'd blocked that out until now. And regarding RFC 2047, I still remember when I first learned about RFC 2231. It really reads like an April Fools joke. Actual text from this RFC:
Character set and language information may be combined with the parameter continuation mechanism. For example:
You (well, the spec writer) forgot the semicolons (it's errata'd). :-)
And now I'm stuck trying to remember off the top of my head whether or not the first few lines are permitted to use quoted-strings in the first place.
With regards to RFC 2047, the worst header I've seen to date is this:
Re: [Kitchen Nightmares] Meow! Gordon Ramsay Is =?ISO-8859-1?B?UEgR lqZ VuIEhlYWQgVH rbGeOIFNob BJc RP2JzZXNzZW?= With My =?ISO-8859-1?B?SHVzYmFuZ JzX0JhbGxzL JfU2F5c19BbXiScw==?= Baking Company Owner
I've been unable to find any possible decoding of those encoded-words that make sense.
... I can't wait until everyone decides to just use UTF-8 for all the headers. Except then I get to yell at everyone who uses Windows-1252 headers right now. >:(
- the charset parameter is nowhere near as correct as you'd like it to be - many RFCs are not followed in practice. In particular, RFC 2047 is literally more often incorrectly implemented than correctly implemented.
I also once recall trying an admittedly weird message (a message whose body is of type message/rfc822) on 4 different IMAP server implementations and getting 4 different answers for part numbering.
I'd say building an email client is harder than building a web browser because at least the people who write the specifications for the latter have been trying to thoroughly document all the idiosyncrasies and how to handle them.
Also, screw CFWS.