What can I do if Magento invoice emails are not sent anymore?

If you use our Automatic Invoice Extension for Magento and invoice emails are not sent anymore please check the following points:
- Please check the configuration at System › Configuration › Automatic Invoice:
Is the Automatic Invoice Extension activated and are the checkboxes for "Generate Invoice" and "Notify Customer" triggered for the specific payment method? - Has the Magento invoice document been generated for the specific order and is it flagged as "invoice email sent"?
- Which Magento Extension is used for the payment method? Perhaps it uses itself a mechanism for generating invoice document and sending the invoice email. Then perhaps it is better to exclude it from the Automatic Invoice generation.
- Are there any error messages in the Magento and/or Automatic Invoice log files in var/log/?