smtp.Address class can't deal with NULL address - breaks bounces

[iko]

The smtp.Address class has changed to always force the existence of a domain
name. This breaks the null address (<>) used to send bounce messages, since it
will get transmogrified into <@your.dom.ain> which is not a valid address.

While there may be reasons to enforce existance of domain name in some places,
this is not the place, since the Address class knows nothing about the use of
the address (To? From? Something else?), and anyway, guessing email domain names
is likely to get you the wrong guess.

This patch changes it back.

[exarkun]

Resolved a while ago