| I'd say that user authentication (requiring auth to send mail "from"
| your domain) and a lookup list of allowed email addresses for each
| account would "secure" it. Note that it doesn't prevent anyone on
| the rest of the Internet from forging your email addresses. There is
| currently no way to prevent that because email is not secure. (DKIM
| helps, but not many recipients require valid DKIM headers, so it's
| possible to fool almost all recipient anyway).
It's worth noting that DKIM by itself isn't something that you (as
a receiver) can use for this. A valid DKIM signature verifies that a
message came from somewhere and hasn't been tampered with in transit,
but a missing or invalid DKIM signature may have many causes and does
not by itself mean that the message is invalid. You need DMARC to go the
extra distance to 'this domain asserts that it only emits DKIM-signed
email and it does not allow its users to send email from elsewhere',
which allows you to conclude that a missing or invalid DKIM signature
means the email is bad.
(Even then you may run into interesting problems with, eg, mailing
lists that alter the Subject: line and add extra text at the bottom
of the message, both actions which will usually invalidate the DKIM
signature.)
Note that the second assertion is a very political decision and has
potentially significant downsides in some organizations.
- cks
