[EAI] Add requirement for punycode conversions in downgrade

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

When sending email from an address containing an ASCII local name and a 
non-ASCII domain to an all ASCII recipient on a non-UTF8SMTP system, a 
downgrade should occur. Following the current EAI drafts, downgrade 
fails if there is no Alternate Address provided for the sender. In this 
case, an Alternate Address is not really required, as the sender's 
address can be represented in ASCII by doing a punycode conversion on 
the domain part of the address. There is also a similar case when the 
recipient address is of the form ascii at utf8.domain.

There are several ways to address these cases:
1. No change to the current drafts. It is the responsibility of the 
sender to provide an Alternate Address if they want Downgrade to occur. 
They can provide the punycode equivalent address as an Alternate Address.
2. The MUA can automatically do punycode conversions for these cases 
when Alternate Addresses are not provided by the user.
3. Downgrade should do a punycode conversion instead of failing because 
of no Alternate Address.

Since attempting delivery is almost always better than failing a 
delivery, I would support adding a requirement to do the punycode 
conversion for these cases in the Downgrade specification.

Ernie Dainow
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