Intermittant SMTP email exception - Service not available, closing transmission channel. The server response was: 4.4.2 service timed out

I have started to see an intermittant error when sending email via my application.

System.Net.Mail.SmtpException: Service not available, closing transmission channel. T开发者_开发百科he server response was: 4.4.2 service timed out.

Now obviously it's timing out for some reason, but any ideas as to why this is happening now? This is the first time I've seen it happen.

Some things to check:

• has anything on the server side changed? Did the admins add some kind of greylisting, perhaps?
• what happens if you connect to the server directly via telnet on port 25?
• does this occur from other applications, as well (e.g. your desktop E-mail client) ?