Mail suddenly refuses to send messages, but receives OK.
Hi. I'm going crazy. Suddenly, today Mail stopped sending messages and periodically tells me so. After eight hours of this, I want to smash something. Frankly, Mail has been cranky for a long time: I know I'm not the only one suffering from the craziness. I've had this issue, occasionally, in the past, but it always fixes itself. Today not. I attach some screenshots to hopefully give someone a clue as to how to solve this.
What I have tried: Logging out of iCloud, restarting the Mac and logging in. Nothing, though it seems to be iCloud-related. Trashing preferences. Nope.
I don't know if this is the correct section to post this, as I couldn't even find a section for Mail. In all honesty, I've been using Apple since the mid-nineties, but I'm losing my faith in them these days. That's another story.
Anyway, OS is 10.13.6.
Many thanks for any help, here.