How to send mail with Sinatra using Pony?

I am having trouble sending email using the contact form I built in Sinatra. I'm using the Pony gem. For some reason I get this error saying 'No such file exist-which sendmail'. I have installed 'sendmail' and I'm still having the same issue. I'm open to any suggestions. The code is below:

Contact form

<form action='/' id='contact' name='contact' method='post' novalidate='novalidate' >

                    <div class="form-group">

                        <div class="input-wrap">
                        <input type="text" class='form-control' name='name' id='name' placeholder='NAME'>
                        </div>

                        <br>

                    <div class="input-wrap">
                        <input type="email" class='form-control' name='mail' id='mail' placeholder='EMAIL'>
                    </div>

                    <br>

                    <div class="input-wrap">
                        <input type="text" class='form-control' name='subject' id='subject' placeholder='SUBJECT'>

                    </div>

                    <br>
                    <div class="input-wrap">
                        <textarea placeholder='MESSAGE' class='form-control' name="body" id="body" cols="30" rows="10"></textarea>
                    </div>
                    <br>

                    <input type='submit' value='SEND' id='button'>
                    </div>
                </form>

main.rb

require 'rubygems'
require 'sinatra'
require 'bundler/setup'



get '/' do 
File.read('index.html')
end

post '/' do
require 'pony'
name  = params[:name]
mail = params[:mail]
subject = params[:subject]
body  = params[:body]


Pony.mail(:to => 'mjstokes1986@att.net', :from => '#{name}', :subject => '#{subject}', :body => '#{body}')

# File.read('index.html')

redirect '/success'

end

get '/success' do
File.read('success.html')
end

Try to tell Pony.mail method where to find your sendmail. You can find your sendmail location using which sendmail. (If this doesn't return anything then you just haven't installed the sendmail correctly).

Pony.mail({
  ...
  :via_options => { :location  => '/path/to/your/sendmail'}
})