Connection refused when using Pony Mail & Sinatra

I am having a lot of trouble getting the Pony Mail gem to work with my Sinatra website (deployed on Heroku). I am aiming to have a basic contact form where a visitor can fill in an email and a subject, their email and phone number and this form will send the information to the clients e-mail.

Here is the code for the form:

<form action="/emailform" method="post">
<p>We'd love to hear from you!</p>
<input type="text" placeholder="Your Full Name" name="full_name">
<input type="text" placeholder="Your E-mail        *required" name="email">
<input type="text" placeholder="Your Phone Number" name="phonenumber">
<input type="text" placeholder="Subject" name="subject">
<textarea rows="5" cols="30" name="comments" placeholder="Comments"></textarea>

<input type="submit" value="Submit" placeholder="Send">
</form>

Here is the code for my actions.rb file (the POST request) (3 dots represent more params/ form fields)

post '/emailform' do
Pony.mail to: 'xxx@gmail.com',
        from: 'xxx@gmail.com',
        subject: 'e-mail from: ' + params[:full_name] + params[:email],
        body: "subject: " + params[:subject] + "phone-number: " + params[:phonenumber]...
redirect '/'

end

post '/homeevaluation' do
  Pony.mail to: 'xxx@gmail.com',
        from: 'xxx@gmail.com',
        subject: 'Home Evaluation E-mail from: ' + params[:full_name],
        body: 'phone number: ' + params[:phonenumber] + params[:comments]...
redirect '/'
end

The error I'm getting right now is

Errno::ECONNREFUSED - Connection refused - connect(2):

I am seeing this by viewing the Heroku logs.

I have been struggling with this issue for a while and can't find any useful information online. If anyone has used the gem or knows of another way to accomplish the same task i'd love to hear it!

This looks like a setup issue. Are you using some kind of Heroku addon for e-mail? We use Sendgrid so we need these lines somewhere in the code:

Pony.options[:via_options][:user_name] = ENV['SENDGRID_PASSWORD']
Pony.options[:via_options][:password]  = ENV['SENDGRID_USERNAME']

Edit: More verbose options

Pony.options = {
  via: :smtp,
  via_options: {
    address:              "smtp.sendgrid.net",
    port:                 "587",
    domain:               "heroku.com",
    authentication:       :plain,
    enable_starttls_auto: true,
    user_name:            "foo@heroku.com",
    password:             "foo"
  }
}