I could not deliver the mail

Hello everyone, I sent an email campaign but I received a lot of reply emails in which there is in the subject “I could not deliver the mail :(”. Looking at the statistics there are no problems and the mail is sent but not I understand if there was actually an error or what it could be.

Thanks a lot to everyone

Probably means it has been sent, just that it was rejected by the mail server at the receivees end. Possibly incorrect email address, however, there should be a rejected code in the bounced back message.