Exchange 2010 to 2016 mail flow stops with "421 4.4 2 connection dropped due to socket error"

Had a client ring today with a mail flow issue.

They are most of the way through their migration to 2016, but mail flow stopped with the error "421 4.4 2 connection dropped due to socket error" on the exchange 2010 side - when trying to relay to exchange 2016. This occurred after patching - but i'm not 100% on which patches they applied - and didn't really have the time to find out.

Long story short - enabled TLS 1.2 on the Exchange 2010 (on a 2008 R2 OS) as per https://support.quovadisglobal.com/kb/a433/how-to-enable-tls-1_2-on-windows-server-2008-r2.aspx

then restarted the transport service - and mail stated flowing again.

While this is known for Exchange 2019 on Server 2019, where TLS 1.2 is the default - i wasn't aware this was being retro-fitted......  not a bad thing.... and its only going to catch out the people that are lagging behind... still, considering how many people are lagging behind - this quick post might help!