Alright so I have been troubleshooting an issue today and seem to have come to a dead end. 

The forums on our DNN site are set up so users can subscribe to each forum to receive email notifications when a Thread/Post is created, replied to, or deleted.  I do not believe any adminstration changes were made before the issue started happening (Used to work just fine). 

Currently if you create a thread on an existing forum only the creator receives an email, and I receive the following log error for each subscriber that was supposed to also receive an notification email but didn't.

InnerException: Service not available, closing transmission channel. The server response was: 4.4.2 Message submission rate for this client IP address has exceeded the configured limit
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: System.Net.Mail.MailCommand.CheckResponse
StackTrace:
Message: System.Net.Mail.SmtpException: Service not available, closing transmission channel. The server response was: 4.4.2 Message submission rate for this client IP address has exceeded the configured limit at System.Net.Mail.MailCommand.CheckResponse(SmtpStatusCode statusCode, String response) at System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, SmtpFailedRecipientException& exception) at System.Net.Mail.SmtpClient.Send(MailMessage message) at DotNetNuke.Services.Mail.Mail.SendMail(String MailFrom, String MailTo, String Cc, String Bcc, String ReplyTo, MailPriority Priority, String Subject, MailFormat BodyFormat, Encoding BodyEncoding, String Body, String[] Attachment, String SMTPServer, String SMTPAuthentication, String SMTPUsername, String SMTPPassword, Boolean SMTPEnableSSL)

I have created a new Forum and tested it with new users, and I do receive the correct email notifications with the new forum. 

The Settings for both forums are the same... so I am unsure what could be causing this issue... Any help would be much appreciated. 

Thanks,