Technical details on how Anti-spam filters work

MX Records

Anti-spam filtering is enabled by changing the domain's MX records to route incoming emails to the filtering servers first before passing the message off to the MX machines for delivery. Domains using non-DreamHost nameservers will need to manually update their MX records to reflect the change.

The filter records are:

  • 0 mx1.mailchannels.net
  • 0 mx2.mailchannels.net

Special considerations for forwarding addresses

There are certain things to remember with filtering in relation to forwarding email addresses. The details for the four combinations with this setup are outlined below:

  • If the domain is being filtered, and the address is being forwarded to another DreamHost email address that is also filtered, email gets sorted based on the filter mail settings of the receiving mailbox address.
  • If the domain is being filtered, and the address is being forwarded to another DreamHost email address that is not filtered, then email will not be filtered.
  • If the domain is being filtered, and the address is being forwarded to an outside address only (like @gmail), then the settings for the forwarding address' domain will be used.
  • If the domain is not being filtered and it forwards to a DreamHost email address that is on a filtered domain, the mail will get filtered.

The following table shows how Anti-spam filtering works for/with forward-only addresses. A@example.com is forward-only and sends emails to B@example.net which is a fully hosted mail account:

A@example.com uses filtering B@example.net uses filtering Is filtering done on incoming mail? What filter is used?
Yes Yes Yes B@example.net
Yes No Yes, but spam emails above score threshold are rejected. Those delivered have scoring headers appended. N/A
No Yes Yes B@example.net
No No No filtering done N/A

Filtering and what it does

There are three services currently set up to filter spam. Below is a description of each:

Vade Secure (soon to be removed)
This is the filter service that scans the incoming messages for spam and other categories such as social or promotional emails. Depending on multiple factors, the message will be tagged with a status (SPAM, SN, etc) to determine if the message should be delivered to the recipient's inbox, or filtered into the Spam folder.  However, spam emails that are beyond a certain threshold will be rejected.
Procmail
This is the delivery agent for the mailbox which actually places the email into Spam, Social or Promotions folders when it detects the Vade Secure headers. This is also responsible for the user defined filters defined in the panel.
Mailchannels (enabled as of Sept 2020)
Mailchannels is a Cloud-based mail filter. It accepts and evaluates mail, adds a specific header for spam messages, then relays messages to MX servers. 

See also

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Article last updated PST.

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