Problem with signup form in Wordpress site

I have tried to make a signup form on a Wordpress-site using a widget on the sidebar. However, when I put in the email-adress, I get the errormessage:

"Invalid Email Address
Sorry, but the email address you supplied was invalid and needs to be fixed.
Click here to go back to the form and make the required changes."

I know for sure the email adresses are correct, I tried several. Any ideas anyone?
My sites adress is

surfacemedia, 6 years ago


I've had this happen on one of our sites too. Not wordpress related but a little strange that no one has actually touched the coding of the site for 6 months and then this feature goes down. Perhaps it's something from CMs end? The web address we're experiencing this on is

Join 200,000 companies around the world that use Campaign Monitor to run email marketing campaigns that deliver results for their business.

Get started for free