I got a weird one that we have been fighting for sometime now. In our Exchange 2013 environment, we are constantly running into SMTP timeout issues when trying to send emails to non-Exchange environments. For instance, when Exchange goes to make that handshake, after a 15 second timeout, the connection just breaks and cant do anything outside of having the recipient white list us. Problem is, this is only happening to one client out of 100 that has the exact same setup. Its the timeout issue and cannot find anywhere to tell SMTP to go beyond that 15 second SMTP and wait until the other end proceeds with the greeting. Any ideas would be greatly appreciated.