You won't know what you are doing wrong until you start reading and displaying the responses from the email server. However, I use smtpcorp's email server, and know that is not the correct port. Use port 2525. edit: And not the correct server.
// change this
AT+CIPSTART="TCP","mail.smtp2go.com",587
// to this
AT+CIPSTART="TCP","smtp2go.com",2525