Mageplaza SMTP extension only connects your mail server and helps make sure your emails are sent to customers' inbox instead of the SPAM box.

The error of duplicate emails may be due to the third module, which affects the check form and recalls the mailing function.

To make sure it's not a bug from SMTP, you can go to app/etc/config.php and disable the SMTP module ( and configuration on the dot-mailler of Magento Core ( If the error does not appear, then it is not an issue of our SMTP module and vice versa.

NOTE: Because the SMTP module and Magento dot-mailler have the same functionality; you can only enable one of them at a time.
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