Spam is a known headache in today's email solutions. While many anti-spam technologies helps, they often mark important emails as spam by mistakes.
Current spam protection use black and white listing which is not effective at all because spammers will always keep changing their IP and email addresses.
After seeing so many spams in our inboxes, I notice that many of them do not have valid sender domain. Thus, I believe I have a foolproof enhancement to filter most of these spams that do not have a valid sender domain - by blocking or rejecting all emails that come from unknown domains.
On the opposite side, we should also block outgoing spam to do our part not to contribute to the spam marketplace. Blocking all emails send out of our Zimbra server that has sender email address faked outside of our hosted list of domains will be extremely effective, especially for Zimbra users who are using infected computers. Has anyone manage to get this implemented?
If not, please vote for this at: https://bugzilla.zimbra.com/show_bug.cgi?id=53852
This will help to ensure that Zimbra email servers around the world are harden and difficult to be target to spam others, unlike Exchange servers.