title: Invalid email when sending through SurveyMonkey API
body: <p>I am trying to use the SurveyMonkey API to send email surveys and I have hit many issues. The latest one is that the API will not allow email addresses with a plus sign (e.g. john+smith@gmail.com). The send_flow response will have the email in the invalid_emails list. I've tried the same email on the website and I am able to add it as a recipient, so this is a validation issue on the API only. 
  This issue makes it very hard to use the SurveyMonkey API to reliably send email surveys. </p>