How is Inbound Email Filtered?

Server-Level

The first level of filtering on inbound mail is server-level; this goes through basic checks for standard protocol compliance, such as:

  • verifying that the remote server is sending valid connection commands
  • looking for SPF records that must be honored
  • checking the sending IP against reputable, third-party real-time blacklists.

The basic protocol compliance checks occur at the initial connection attempt, before any email header information is accepted (the header contains the ‘To’ and ‘From’ addresses, as well as the subject line and other message data), so protocol failures (such as FCrDNS) will only have the sending IP address logged. This means that we will not be able to locate those failures when searching for a sending or receiving email address, only the sending IP. This is why it's very important that senders always bring those rejection notices to their email admins for review. In the case of RBL and SPF checks, however, header information is received by that time, so we would have records of the sending domain in the email logs in those cases.

If mail is rejected at the server-level, a rejection message will be returned to the sender with the reason the message was rejected so that they can provide those messages to their email administrators. Failures at this level are sending-side issues that would need to be addressed by the sender’s host.

Please note, while some of these checks do help with filtering spam based on invalid protocol and IP reputation, they are not intended to be the main spam filtering for your accounts.

Additional information about RBL checks:
http://support.hostrocket.com/knowledgebase.php?action=displayarticle&id=535
Additional information about FCrDNS checks:

http://support.hostrocket.com/knowledgebase.php?action=displayarticle&id=533


Account-Level

So long as messages pass those basic server-level checks, they will then be processed according to any spam filtering settings you have configured in the cPanel, such as general SpamAssassin / Mailscanner scoring / filtering, and Account- or User-Level Filters (in the paper_lantern theme, Account- and User-Level Filters are named Global Filters and Filters, respectively).

Depending on the server your email is hosted on, your spam filter will be either SpamAssassin or MailScanner. Either option is meant to be a general filter that assigns a score to a message based on certain spam characteristics. The more spam characteristics a message matches, the higher the spam score, so the higher the score, the more likely a message is spam.

Keep in mind, SpamAssassin and Mailscanner are not setup to filter mail by default, only scan. Setting up filtering rules would need to be done by the customer in the email administration settings. Some periodic adjustment to the settings will be necessary to tailor the filtering to your individual needs; some customers receive mail which would require the scoring to be more or less strict than others, so there is no one-size-fits-all configuration.

After the message passes through the general spam scoring filter, Global/Account-Level and User-Level filters would apply. These can be used to configure more specific filtering rules, such as moving mail that gets the {Spam?} / ***SPAM*** tags to a specific folder, or discarding messages with certain phrases in the subject or body of an email. These filters have a wide range of options, guidelines for which are covered in the cPanel documentation:

cPanel - How to Configure Mail Filters
cPanel - Global Email Filters
cPanel - Email Filters

In addition to the filtering on our mailserver, your mail program may potentially have its own spam filtering / routing preferences. Junk/Spam filtering by your mail client is not configured or controlled through our service, so you will want to refer to your settings and/or the documentation for the program(s) for those adjustments, if necessary.

 

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