Google Apps adds DKIM support

[encyclo]

I've just implemented this, it's really easy to do:

Spam takes another hit: email authentication now available [googleenterprise.blogspot.com]

Since we launched Gmail in 2004, we have supported email-signing standards such as DomainKeys and DomainKeys Identified Mail (DKIM) to help validate outbound mail with digital signatures. (...) we’re making it possible for all Google Apps customers to sign their outgoing messages with DKIM, so their sent mail is less likely to get caught up in recipients’ spam filters.

Implementation details:
Authenticate email with a domain key [google.com]