Setting up the SMTP or REST API sending for emails in GD Mail Queue is not very difficult, but it still can fail to send emails. Why? Different services might have different rules related to registering email addresses, use sandbox and other issues that are outside of GD Mail Queue, and you need to make changes to the service you use. But, in many cases, GD Mail Queue can help you with debugging the issues if the sending is not working.
GD Mail Queue (Free and Pro) have two very powerful tools to debug and track various sending issues. First, there is an email sending log and second, there is an email sending test tool. Email sending log can log email sent directly, and emails sent through the queue, and it contains all relevant information about the email, including the errors that were returned by the SMTP service or REST API engine. The error returned depends on the service you want to use, and in some cases, you will get the message that email was sent, but still, it doesn’t get delivered. GD Mail Queue can control the process up until it gives the message to the service, and at that moment service can say if all is OK with the message and in most cases, that means it will be delivered. But, there are some specific cases where the GD Mail Queue reports that email is sent (and indeed it is, it is sent to the service, but it has no control what service does after that, and it has to trust the service about the delivery). If this happens – you get the green OK message, but the email is not delivered – you need to contact the support for the service you use about the problem.
Email Test Tool
Email testing tool is a more direct method to test email delivery. It will get the result shown immediately, but the same goes as with email log: error or success message are depending on the service used, and there is no guarantee that service can say that it is OK and email doesn’t get delivered: that is the service issue, not with the plugin.
Currently, the test tool allows you to add CC and BCC fields, in the future will include attachments and custom headers too. And, there are two typical cases, all is OK, looking green and the error in red.
The Gmail or Amazon SES gives very detailed error messages that usually point to exact problem, as seen here with the Amazon SES service and the problem with email not being verified.