Error: Mailbox unavailable. The server response was: 5.7.1 [126.96.36.199] The IP you’re using to send mail is not authorized.
Description: This error occurs while sending mail . This information [188.8.131.52] holds our system IP Address. While using GMAIL SMTP settings we will get this error and the error is not because of GMAIL but SMTP configuration is not properly completed. The cause of the error, while sending a mail, the server will look for the configured from address to send the mail and if that information is not provided, Mailbox unavailable error will throws up. If the from address is not provided then, then by default the from address will be taken from IIS SMTP settings.
Solution: message.From.Add(“email@example.com”); or configure the SMTP settings with all required information’s.