When rendering the parameters need to go in render_options rather than output_options.

Email doc (test_email_params.Rmd):

---
title: "Test email parameters"
output: blastula::blastula_email
params:
    email_name: "John Smith"
    other_param: "App abc"
---

Email content...

Email is for `r params$email_name`

Create email:

blastula::render_connect_email(input = "test_email_params.Rmd", 
                       render_options = list(params = list(email_name= "Willy Wonker")),
                       connect_footer = FALSE)

Note: Not all parameters need to be passed into an RMarkdown doc, anything not passed in will use t default values.