The wizard at pobox.com is pretty good, but
[...]
In short, there are sloppy people out there and it doesn't help matters at all.
The reason this sloppiness can proliferate more than for example when
publishing a web page, is that the publisher gets no feedback from users.
The recipients (verifiers) have no incentive or permission to give feedback.
This is a structural problem which IMHO must be addressed structurally.
Education alone is not enough, because there is no structural incentive. For
example, the spec could be modified to allow a validator to e-mail the domain
(at "postmaster(_at_)domain"?) periodically (weekly?) if there are syntax
errors.