STARTTLS extension is not supported by server error after installing postfix

I have written a mailer which is using AWS SES.

The code snippet looks like:

import smtplib

content = 'This is a test email'
mail = smtplib.SMTP("smtp.123-reg.co.uk", 587)
mail.ehlo()
mail.starttls()
mail.login('bookings@polestarstudios.co.uk' '123456')
          mail.sendmail('bookings@polestarstudios.co.uk','davidjohnshepherd@msn.com',content)
mail.close()

The piece of code was working fine till yesterday. Then I installed postfix on the machine. The mailer stopped working after that and it is throwing STARTTLS extension is not supported by server exception.

What could go wrong? Do I need change any config file?

Thanks.