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

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!

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