I've written a spec for the subaddressing format which many people use.
Objections:
1. ``+'' is not the de facto standard; qmail's ``-'' is in wider use by
several measures. (The separator is configurable by the sysadmin, but
most people stick to dash, which I recommend for a variety of reasons.)
2. The requirement that foo-bar be delivered to foo ``by default'' is
violated by qmail. Of course, I make it easy to set that up; but I think
that bouncing is the right default.
3. The requirement that foo-bar be delivered to foo ``by default'' is
violated by any delivery agent that doesn't support subaddressing. What
is the point of a requirement that nobody can rely on?
4. The requirement that foo-bar be delivered to foo ``by default'' is a
violation of the as-if principle. Again, what is the point of a
requirement that nobody can rely on?
5. The ``canonically quoted'' section confuses content with encoding.
Any delivery agent that uses your syntax is almost certainly violating
RFC 822, section 3.4.4. The correct rule is very simple: any nonempty
string of ASCII characters is a valid local part. RFC 821 and RFC 822
specify encodings of those strings for SMTP and mail headers.
6. Your restrictions on MUA quoting outlaw some valid addresses.
7. Your restrictions on MUA quoting produce incorrect results for _all_
quoted addresses when the MTA handles quoting correctly. MMDF and qmail
handle quoting correctly.
8. ``MUST ignore subaddresses for the purposes of such validation'' is
bizarre. If an MUA understands subaddresses, why shouldn't it validate
them?
9. The requirement that list servers ignore subaddresses effectively
prohibits the use of cryptographic cookies in return-path addresses for
sender authentication.
10. ``Delivered to that primary address'' is unclear.
11. ``Owner of that primary address'' is unclear.
12. ``Rewriting'' is unclear.
13. ``Folder'' is unclear.
14. ``Command line interpretor'' is unclear.
---Dan
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