Unable to send e-mail to Gmail or Hotmail with Subscribe By Email plugin

I have installed the Subscribe by email plugin successfully, but am unable to send confirmation e-mails to Gmail or Hotmail accounts. I have managed to send a confirmation email to an address on the same domain the website is hosted though.

In one of your other support posts, you recommended the Easy WP SMTP plugin. I installed this plugin and tested the connection. Emails could not be sent to Gmail or Hotmail addresses but I was able to send it to an address on the same domain as the website.

Please advise what to do next.