Running the latest 16.85.1 and still received the dreaded message in a red box "Message Failed to Send". It's not every email, its's occasional and seemingly when I reply to an email chain, that has embedded images (from signatures) being shown as attachments, (although I will watch this more closely).
If I delete the attachments (on the failed email), it still will not send. I can do this over and over with the same email. It will fail.
If I take the original email chain I replied to, and remove all the attachments first and then send - it will work
If I take the same email chain and start/create a new email where I simply paste the entire contents of the failed email, but none of the attachments, it also then sends.
Almost as though once it decides it does not like the email, there is nothing you can do to make it send.
As many have said this is a new phenomenon and I have now run into issues, because of an email I was 'supposed' to send that I did not, only to find it in my drafts and for a while, blaming myself for not hitting send, until I realized that I did hit send, but it actually will not send.
Now I have to check upon sending email that the 'red box' doesn't appear.