Read this if your wordpress site is hosted by one.com

Hi

There are limitations on 20-25 mail per day if you do not use a real address created on the domain account. In order to fix this you must:

1:)
Create an adress postmaster@[yourdomain] at one.com’s adminstration pages.

2:)
Then you have to put the same e-mailadress in the “Sender’s e-mail address”-field in the settings of Simple Subscribe.

Then it should work just fine.

/Anders

https://www.ads-software.com/plugins/simple-subscribe/