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Trend Micro Email Security supports DKIM signing for all outgoing messages from a specific domain. Recipients can verify that the email messages from the domain are authorized by the domain's administrator and that the messages, including attachments, have not been modified during transport.
The DKIM signing settings apply only to the selected sender domain.

Procedure

  1. Go to Outbound Protection → Domain-based Authentication → DomainKeys Identified Mail (DKIM) Signing.
  2. Click Add.
    The Add DKIM Signing Settings screen appears.
  3. Select a specific sender domain from the Managed domain drop-down list.
  4. Select Enable DKIM signing.
  5. Optionally select Sign email messages with no envelope sender addresses.
    For email messages with no envelope sender addresses (such as auto-reply messages or bounced messages), Trend Micro Email Security attempts to find the sender domain from the email header From and applies DKIM signing settings of the sender domain.
  6. Configure general settings for DKIM signing.
    • SDID: select a signing domain identifier from the drop-down list.
    • Selector: selector to subdivide key namespace. Retain the default value.
    • Headers to sign: select one or multiple headers to sign and customize more headers if necessary.
    • Wait time: specify how long it takes for a key pair to take effect. Trend Micro Email Security starts to count the wait time once it finds the public key in the DNS.
    • Key pair: select a key length and click Generate to generate a key pair.
      Note
      Note
      Use the generated DNS TXT record name and DNS TXT record value to publish the public key to your DNS server.
      If your domain provider supports the 2048-bit domain key length but limits the size of the TXT record value to 255 characters, split the key into multiple quoted text strings and paste them together in the TXT record value field.
  7. Configure advanced settings for DKIM signing.
    • Header canonicalization: select Simple or Relaxed.
    • Body canonicalization: select Simple or Relaxed.
      Note
      Note
      Two canonicalization algorithms are defined for each of the email header and the email body: a "simple" algorithm that tolerates almost no modification and a "relaxed" algorithm that tolerates common modifications such as whitespace replacement and header field line rewrapping.
    • Signature expiration: set the number of days that the signature will be valid.
    • Body length: set the number of bytes allowed for the email body.
    • AUID: specify the Agent or User Identifier on behalf of which SDID is taking responsibility.
  8. Click Add to finish adding the DKIM signing settings.