Relenta provides application-wide mail-merge by any of the contact data fields (see How do I personalize email messages topic for additional details).
These mail-merge macros can be used in not only in canned responses, but also general compose window, mass-email to group of contacts, email campaign templates, and so on.
NB! When you use macros in canned responses, they will be displayed in the compose window unparsed, i.e. [FIRST], [COMPANY], etc. They will be replaced with the actual values only after you click Send.