I have a form. When a user submits the form after filling it, I have to send a mail to the user by using getresponse.com and also the user email should be added to my mailing list. I have an account in getresponse.com.
The getresponse github page has examples of using their API in C#, Javascript, Python, Perl, and PHP. You don't tell us which language you want to use -- is it none of these?