There are a number of front-line tests that are very effective at blocking spam that are disabled by default because they will not suit every configuration. These tests are very effective at blocking spam and we strongly recommend enabling them if possible. The tests are:
- HELO tests
- Reject Unknown Sender Domain
Greylisting is an effective spam blocking test. When enabled it will reject all mail temporarily. All SMTP compliant mail servers will defer the mail and resend it after a set period of time, usually around 5 minutes. On the other hand spam sending servers are rarely SMTP compliant, it is very likely that they will not resend the rejected mail so straight away spam will be blocked. If a spam server does resend the mail it is likely that the spam server IP address or the content of the mail will be blacklisted when the mail is received a second time and a different test will block it. Greylisting comes with an auto-whitelisting feature. Mail servers that send you mail regularly will be whitelisted after the whitelisting settings are met. Greylisting is enabled and configured in System Setup > Mail Relay > Greylisting.
The Sender Policy Framework (SPF) is an open standard specifying a technical method to prevent sender address forgery. More precisely, the current version of SPF — called SPFv1 or SPF Classic — protects the envelope sender address, which is used for the delivery of messages. You do not need an SPF record for your own domain to use the SPF test on SpamTitan. The SPF test will only be applied to mail from domains with SPF records. SPF is enabled in System Setup > Mail Relay > Sender Controls.
The HELO tests are performed on the HELO host name provided by the sending mail server. There are two tests that can be carried out on this hostname:
- Require Fully Qualified Hostname
- Require Resolvable Hostname
Many spam servers do not meet these requirements. If one of your customers mail servers does not meet these requirements you can add their HELO host name to the Allowed HELO Hostname Restrictions list (this list will appears once either of the HELO tests are enabled). These tests are enabled in System Setup > Mail Relay > SMTP Controls.
The Reject Unknown Sender Domain test examines the domain of all inbound mail. It verifies that the domain has either an A or MX record. If it has neither the mail is rejected. For example if you received a mail from firstname.lastname@example.org a DNS check would be performed to see if xyz.com had either an A or MX record, if it has neither the mail would be rejected. This test is enabled in System Setup > Mail Relay > SMTP Controls.