I have E-mailed my hosting provider to check if it is a configuration issue on their side:

Waitng for a reponse from them...

"I am encountering an Error when trying to send e-mail newsletters via our ‘DotNetNuke’ based web site.

This used to work in the past. 

Error Received:

“There has been an error trying to send the test email. The error is:
Failure sending mail. Unable to read data from the transport connection: net_io_connectionclosed.”

Checked the DotNetNuke forums and this does not seem to be a common problem (apart from a single other user that has logged this issue.) 

However I did come across the following while looking for a solution on Google: 

http://www.systemnetmail.com/forums/forum_viewpost.aspx?forum=2&post=841

Shortened for brevity below… 

“Swapping out hardware on server running Server2003/IIS6 to new server with same setup. But, on the new server, when I test one of my asp.net 2.0 emails, I get the following error…”

“System.Net.Mail.SmtpException: Failure sending mail. ---> System.IO.IOException: Unable to read data from the transport connection: net_io_connectionclosed.” 

And response: 

“Gheesh, figured it out finally.
Had to change smtp to all unassigned.
Then make sure that IIS-->smpt-->acess-->Access TAB-->Relay-->only the list below-->Add-->ip Address and granted..
I'd been trying so many things, I guess this last couple had gotten switched. So sorry - hope this helps someone else.”

Now I am not 100% certain that our problem is the same, but it may be an IIS Issue.

If memory serves me right you did upgrade some of your servers a while ago.

I have not sent any newsletters via the site in quite a long time, so I cannot say exactly when the problem started. 

My SMTP Server settings are as follows on the web site:

SMTP Server                      :               xxx.xxx.xxx.xxx

SMTP Authentication     :               Basic

SMTP Enable SSL              :               false

SMTP User Name            :                xxxxxx

I have checked that the password I am supplying is correct by logging into the Mail portal direct on this account.

Could you please check if this is possibly configuration issue on your side.

Kind Regards,"