Hop count exceeded - possible mail loop

Question / Problem

What causes the error "Hop count exceeded - possible mail loop"?

Answer / Solution

Every server has a maximum count for 'hops ' of an mail. (hop count = count through how manyservers a mail was send). Usually that maximum is not reached. But when it 'sreached the server either bounces that mail or does not accept it. So your ExchangeServer does not accept it because it already passed too much SMTP servers.
Check your accepted domains in the settings of your Exchange Server. If one of them is set to "Relay" change it to "Authoritative".