Subscribers to my mailing list do not receive any confirmation email right now

Hi,
Subscribers to my mailing list do not receive any confirmation email right now. It was working fine, but after few subscribers the email confirmation message sending process stopped. When I tried to check and see the problem I got this error message “There was a problem with your sending method. Please check if your sending method is properly configured”, I revised all settings, nothing changed and could figure out the issue. Please help.

Best Regards

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