Error: Could not instantiate mail function on 1&1 servers
If your website is hosted at 1&1 (https://www.1and1.com) servers, and your MailPoet is configured with these options below:
...and you are getting this error message on your Newsletter pages:
...that means you are using an email address which doesn't match your website domain.
To correct this problem, you must use an email address from the same domain as your website (this email account can be created inside your 1&1 control panel) and then specify this address in three places:
1) MailPoet > Settings > Basic tab:
2) MailPoet > Settings > Sign-up Confirmation:
3) On the latest step of the newsletter creation process:
*this must be done for all future newsletters
With these settings correctly setup, you'll be able to send your MailPoet emails using 1&1's servers.