Let’s look into some of the anti-spam strategies in Zimbra. At Bobcares, with our Server Management Services, we can handle your Zimbra issues.

Anti-spam strategies in Zimbra

We will discuss 4 anti-spam strategies used in Zimbra in this article.

Anti-spamming using Postscreen

Postscreen, a type of inbound email firewall, is now available with Zimbra’s Postfix. It effectively blocks bad actors while putting minimal strain on the Zimbra server.

Postscreen has excellent resource management. We can implement it with a small amount of RAM, disc space, and CPU time. But, we advise sticking to blacklists with extremely low known rates of “false positives.”

Anti-spamming using Postfix DNS Protocol Checks

Any external email servers that attempt to send you email should act appropriately and be set up in accordance with public and acknowledged standards. Most aren’t, and frequently for acceptable causes. The Admin Console’s Home >> Configure >> Global Settings >> MTA settings in Zimbra’s Postfix let you do a number of these checks, however all but two of them can frequently produce false positive results.

The zmprov mcf +zimbraMtaRestriction reject_unknown_sender_domain check makes a number of particular tests to guarantee that the sender’s domain genuinely exists in public DNS. The email is rejected if the sender’s domain is invalid.

Anti-spamming using Bad Sending Servers

Now that Postfix is in use, blocklists are being used. There are two types: “RBLs,” or left-hand-side blocklists, and “RHSBLs,” or right-hand-side blocklists. RHSBLs list domains, while RBLs list IP addresses. Regarding how IP addresses and domains are added to blocklists, various blocklist providers have differing ideas. Some plainly use quite harsh listing approaches and readily accept they will occasionally produce some “false positives.” Others lack a clear set of guidelines for delisting. Therefore, we should ensure that the blocklist’s culture matches our company’s sensitivity to false positives before deciding which blocklists to utilize.

These blocklists block the spam from known bad IP addresses.

Anti-spamming using Certain Attachments

Zimbra does not by default filter out email attachments that can be harmful, such as Visual Basic scripts and Windows Registry changes. Therefore, we must block those attachments and inform the recipient and the system administrator that we have done so. Zimbra by default also disables encrypted attachments.

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Conclusion

We’ve provided a brief description from our Tech team on the anti-spamming strategies on Zimbra.