Could not instantiate mail function

Hello! Suddenly a problem appeared. The administrator does not receive a letter about a new subscriber. The letter comes only to the subscriber. I did not change anything, did not update anything. It's just that since Monday night the letters stopped coming. The test letter from Joomla works. Forms created in other plugins also work. Only Membership gives this error and does not send the letter. Please tell me how to fix this error?

Hello

Could be because the mail server could not deliver email to administrator email address for some reasons

Please change Notification Emails to use a different email to see if it works ?

Regards,

Tuan