Canonical Maps allow the server to translate a non-publicly addressable internal domain name into an addressable public domain name.
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Using the example in this document, the server is located in the corp.guardiandigit-al.com domain. If we have a user nick the e-mail address would be nick@corp.-guardiandigital.com. However in our example corp.guardiandigital.com is in the non-policy addressing IP space of 192.168.50.0/24. So on smtp.guardiandigital.com we define a canonical mapping for this domain to translate to the real domain of guardiandigital.com (209.11.107.14). Doing this makes the translation of nick@co-rp.guardiandigital.com to nick@guardiandigital.com publicly addressable.
Once Canonical Maps are enabled the source domain and destination domain needs to be defined for every non-publicly addressable internal domain hosted by mailbox.corp.guardiandigital.com in which users will be sending mail to the Internet for the above example to be complete.
You need to enable Header Filters before using the canonical maps. This can be done in the Header Filters page accessible from the Mail Filters section in the Content And Policy Enforcement (CAPE) center of the Secure Mail Suite. You don't need to create any entries in the Header Filters section.