Whenever you get a connection error when sending via SMTP like The most common reason for this problem is that the ESP (Email service provider) you have defined in your delivery settings doesn’t accept one of the used email addresses.
If you’re sending your campaigns, you’ll sometimes get subscribers marked as errors.
In some rare cases, Mailster shows this error after an Ajax request doesn’t respond to a valid JSON: This error is often caused by third-party plugins or some unexpected server outputs. Please open the developer console to see the problem with this error.
The Browser Console contains important information on Errors. While most of the errors shown are JavaScript-related related some server-related information can get displayed as well.