Hi,
Sorry, we missed your ticket earlier. For any further queries, please open a support ticket in our dedicated support channel.
Can you please check if you have another SMTP plugin installed on your website? If so, kindly deactivate that plugin and hopefully, your problem will be resolved.
Also, there could be other plugins that are blocking the default PHP mail. So, it is recommended that you temporarily deactivate all plugins except Fluent SMTP and then check if the issue got resolved. If this resolves the issue then kindly reactivate all the plugins one by one and check which plugin is conflicting.
Finally, please check if your credential is correct. If your credential is not correct then you will get this error.
Thank you