Unexpected problems with SMTP server

No connection could be made because the target machine actively refused it 127.0.0.1:25

After applying windows update on our server and restarting the machine, we started having problems with our SMTP server. Applications sending emails started producing error - No connection could be made because the target machine actively refused it 127.0.0.1:25

On investigation, it was found that the SMTP server had not started. I tried starting it in IIS 6, but it would not start.

There was an error in windows event viewer - "SMTP could not connect to the DNS server 'xx.xxx.xxx.xxx'. The protocol used was 'UDP'. It may be down or inaccessible."

Now, when I ran ipconfig /all on the server, it gave 2 IP's of DNS server. The one above and yy.yyy.yyy.y
When I tried to ping xx.xxx.xxx.xxx. It was not connecting
So I tried pinging other IP yy.yyy.yyy.y and that worked fine

I changed SMTP configuration to use the second IP address. Refer this document for finding out how to set the domain name server http://technet.microsoft.com/en-us/library/bb124221(v=exchg.65).aspx

After the change, the SMTP server started fine. The errors went away.

Now I need to find out what is going on with the server providers network setup