Recently I had to help a customer determine why he could not relay outgoing mail through his ISP. It just suddenly stopped working earlier this week. To determine why, I asked him to turn on SMTP Client in the Debug Log.
He was puzzled by that. "My Kerio is a mail server and so is the ISP machine. Why do you want to turn on Client?"
He's correct: Kerio Connect certainly is a mail server - it receives mail and serves that mail to clients. But when it SENDS mail, it acts as an SMTP client. As it is the sending of email that was failing, that's what I needed to see in the Debug log.
So it's simple: if you need information about mail leaving your server, turn on SMTP Client. If it's mail coming in that you need to examine, turn on SMTP Server.
In this case, his password had been changed. That showed up quite plainly in the Debug output and was easily fixed by having the ISP reset it.