[EAI] double angle brackets

[Ernie Dainow <edainow at ca.afilias.info>]

One of the key goals in simplifying Downgrade is to drop the double 
angle bracket notation, due to compatibility and security concerns. 
There have been a few email threads on this. I think the following 
summarizes the current thinking.

Dropping double angle brackets on Recipient addresses is part of the 
downgrade simplification to not support downgrade of forward pointing 
addresses (considering these cases as configuration errors).

To replace double angle brackets for the Sender, John Klensin proposed 
using multiple From fields. SM amended this with Reply-To to specify the 
preferred address, as in
       From: EAI-addr, ASCII-addr
       Reply-To: EAI-addr
where the EAI-addr gets dropped in Downgrade.

I did some (non-EAI) tests of multiple From fields with Reply-To on two 
different email systems and both worked as expected. This looks like an 
elegant solution.

SM raised some possible problems with multiple From addresses on mailing 
lists. The suggestion was to address these issues in the mailinglist 
document.

There still needs to be a mechanism to carry the alternate address of 
the sender in the envelope through SMTP relays up until the point that a 
non-EAI transport is encountered. No change is being proposed to the EAI 
SMTP extension for the ALT-ADDRESS parameter on MAIL FROM. If a non-EAI 
server is encountered, Downgrade is done on the message and the EAI 
address in the Envelope is replaced with the ALT-ADDRESS. However, we 
would drop the ALT-ADDRESS parameter on RCPT TO.

   -Ernie Dainow