The advanced email tool workflow component, Create email message text from a template is able to convert an email template into an html payload, where all variables has been substituted. This can e.g. be used, if you want to verify the contents of an email without sending any email.
The component has this parameter:
Generates the HTML document for an e-mail body, based on an Apache Velocity template. The template must be uploaded in the "Mail templates" folder in the library.
In the Velocity template, you can use variables that will be replaced with metadata values (technically Camel header values). The "Set workflow variable" component can be used to set the values for the variables that are used in the mail template.
Note that Velocity variables must start with an alphabetic character (a .. z or A .. Z). The rest of the characters are limited to the following types of characters:
Alphabetic (a .. z, A .. Z)
Numeric (0 .. 9)
Hyphen ("-")
Underscore ("_")
For additional details of the Velocity format, see https://velocity.apache.org/engine/1.7/user-guide.html