If an email reply from the thread view failed to send — say, because of a domain verification issue or a contact with no email address — it looked exactly like a success: the composer cleared, the message quietly vanished, and nobody was told anything.
Now a failed send behaves the way you'd expect. You get an error message explaining what actually went wrong, the pending message is rolled back, and your text is restored to the composer so nothing is lost. Successful sends are unchanged.
Live for all workspaces — nothing to do on your end.