Solved: postfix lost connection after EHLO on RHEL7

I needed to get some software from a lab that only distributes the software as .zip attachments by email. Let's ignore the wisdom of choosing that distribution method in 2016 (nowadays most mail setups strip out zipped attachments) and instead focus on the problem at hand.

I set up a local account on a RHEL7 server running Postfix. When the message was sent to me, this is what appeared in the logs:

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