Hi There!
Since moving our systems to Vodafone Internet (The only thing that changed), we have started to get smtp 420 tcp errors on our logs for our mail server to ISP's like xtra, and gmail for some reason. It was suggested we try adjusting the MTU, which was 1492, but we subsequently changed to 1500, due to web browsing issues. This fixed all the email problems for a day, now we only have issues with some people rather than everyone.
What is the correct setting and why would I be getting 420 TCP errors?