Message: System.Net.Mail.SmtpFailedRecipientsException: Unable to send to a recipient. ---> System.Net.Mail.SmtpFailedRecipientException: Mailbox unavailable. The server response was: 5.7.1 Unable to relay --- End of inner exception stack trace --- at System.Net.Mail.SmtpClient.Send(MailMessage message) at DotNetNuke.Services.Mail.Mail.SendMailInternal(MailMessage mailMessage, String subject, String body, MailPriority priority, MailFormat bodyFormat, Encoding bodyEncoding, IEnumerable`1 attachments, String smtpServer, String smtpAuthentication, String smtpUsername, String smtpPassword, Boolean smtpEnableSSL)

I've been able to send out an email whenever a user completes the form. All of sudden, this errors occur. Emails can be received internally, but when a users enter an email from outside (gmail, outlook, etc...), this occurs. Does anyone seem to know the problem?

Solved the problem. IP changed.