I anybody else having this problem. I cannot send even the simplest of messages this morning. I consistently receive this warning: whenever I try.
I am not having any such problems with my iCloud server so I do not think its a snag with Apple Mail. I cannot find an obvious way of asking bt about this!
Solved! Go to Solution.
I have got my email working again. I am not quite sure how/why but it may help if I describe the actions that I took to get it working again. It seems that my account SMTP setup was changed (without my input). When I ran Connection Doctor (available in Mail/Window/Connection Doctor) my btinternet connection showed green (OK) but I noticed that the SMTP connection details said "login not required". I selected my btinternet IMAP account in the accounts window of the preferences panel, selected "Server Settings" and unchecked the "Automatically manage connection settings" checkbox in the SMTP section to open up the SMTP settings dialogue. The "Port" was set to 587 (correct) and "Use TLS/SSL" was checked (correct) but "Authentication" was set to "None". I changed "Authentication" to "Password" and clicked on "Save". I exited and restarted Mail and was then able to send emails again. (The fix did not work until I had quit Mail and relaunched it.)
I have the exact same problem. It's very annoying and slightly weird. The problem is also affected by the addressee. I can send test messages to my iCloud e-mail but not other addresses. I thought that I had fixed the issue last night by resaving my IMAP SMTP settings and restarting Apple Mail but the problem has reappeared this morning. It's reassuring not to be alone. I just hope Andy_N's observation that "the issue can clear itself after a day or two" proves to be the case. If not, how do we get a moderator involved?
Although port 587 will usually work, port 465 is the recommended port. It might be worth changing your SMTP port to see if that helps.
As an experiment, can you try a different mail client other than Apple Mail on one of your Macs and see if that works.
Just one other thing that might be relevant. I have had an empty VIPs mailbox suddenly appear in the sidebar of Apple Mail.
Its probably not directly relevant to your problem, but who knows with Apple!!