Merge tags aren't supported for SMTP. If you want to have most of your content stored in the template and replace multiple pieces of information for each recipient, you'll need to use the API. You can use merge tags, and set a default value and then per-recipient values using global_merge_vars and merge_vars:
http://mandrillapp.com/api/docs/messages.html#method=send-templateIn your template, you might have something like this:
Dear *|NAME|*,
Your username is *|USERNAME|*
You'd then use global_merge_vars to set a default value, and merge_vars to set recipient-specific information . In the above example, the names of the merge_vars are NAME and USERNAME.