I’d bet dollars to donuts — a no longer sensible comparison — that this is bad programming, though not necessarily by Medium.

It’s trivial to write code to go through a list of email addresses and send your post to them. The code simply makes a connection the server that handles mail for the domain and writes out the message. Repeat for each email in the list.

But that simple approach ignores reality. Sometimes you can write out the message but the server has some problem and spits back an error code. Instead of saying OK, it says Nope, can’t right now.

If your code doesn’t check for errors and loop back later to try again, that bit of mail never gets sent.