Unable to telnet to localhost after CU5

Hi

We upgraded E2013 to CU5 on Sat 13 - On Thu 18 at 9:12 all 4 CAS servers stopped processing mail

error from smarthost:

421 4.3.2 The maximum number of concurrent connections has exceeded a limit, closing transmission channel

421 4.3.2 Service not available

netstat -an | find "25" shows port is listening

Test-SmtpConnectivity shows success

Any ideas appreciated

shapi