13/21 here. Mostly got hung up on several “this was valid in earlier RFC, and later removed” kind of situations. There are several where I picked the correct answer, but where I know many websites that won’t accept it as valid, and that’s not even the more esoteric ones.
But they will work, and according to the spec, you have to build your system so that it can handle those cases. Obsolete doesn’t mean incorrect or invalid, just a “you shouldn’t do this any more”.
Obsolete Syntax
Earlier versions of this standard allowed for different (usually more
liberal) syntax than is allowed in this version. Also, there have
been syntactic elements used in messages on the Internet whose
interpretation have never been documented. Though some of these
syntactic forms MUST NOT be generated according to the grammar in
section 3, they MUST be accepted and parsed by a conformant receiver.
Some of those “obsolete” things are outright blocked for specific reasons. For example, routing addresses through multiple servers. It was abused by spammers, so it’s almost always denied these days.
13/21 here. Mostly got hung up on several “this was valid in earlier RFC, and later removed” kind of situations. There are several where I picked the correct answer, but where I know many websites that won’t accept it as valid, and that’s not even the more esoteric ones.
Yeah I feel like the correct answer for anything obsoleted by a more recent RFC should be “Invalid”.
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But they will work, and according to the spec, you have to build your system so that it can handle those cases. Obsolete doesn’t mean incorrect or invalid, just a “you shouldn’t do this any more”.
https://datatracker.ietf.org/doc/html/rfc2822#section-4
Some of those “obsolete” things are outright blocked for specific reasons. For example, routing addresses through multiple servers. It was abused by spammers, so it’s almost always denied these days.
Looks like this:
<@foo.example.com@bar.example.com:123@example.com>