Mail will not send - OS Sierra

My final shot: delete the mail account completely; reboot in safe mode (hold down shift while booting); reboot; setup a new mail account.

If that fails, I'd be trying https://getsupport.apple.com/

I have the same problem with Mail on my Macbook Air. Ever since upgrading to Sierra, my Mail will not work properly with my non .me.com addresses.
I can use Microsoft Outlook for my non .me email but it refuses to function with my .me account.
So I have to use Outlook for most of my email and Mail for my .me email. Not ideal and very frustrating.
I have tried deleting email accounts and reinstalling them but to no avail.
Please can Apple explain why a programme that they supply with every Mac fails to work after an OS upgrade?
I am not upgrading my desktop until I know that this problem is fixed.

I have a similar problem (MBP, Sierra). In my case, it is with my Yahoo account. I can receive, and delete emails. I cannot send any email. My 'Outgoing mail server' shows 'offline' for Yahoo. When attempting to manually add a Yahoo SMTP server, and then try to use it, Mail simply deletes the added entry from the list of server, and I am left with the 'Yahoo! (offline)' server.

I have a gmail and iCloud emails and both work fine (so far).

I would appreciate any suggestions. Thank you.

I have the same problem. About two weeks ago, I spent four hours on the phone with Apple support, bouncing from one "senior technician" to another, and they couldn't guess any better than I can. Here's the thing: I bought a brand new iMac today, did NOT use the Migration Assistant, just logged into iCloud during setup, fired up Mail, it downloaded all my messages fine, but when I tried to send an email, I got the same error! This leads to me believe that the problem is not on our boxes, it's on the far end (at the server/mail exchange, whatever, I'm not that familiar with the ins and outs of email). Maybe this tidbit helps?

I had this identical problem, albeit with AOL rather than iCloud.

For some reason when you add a new email account using one of the first 5 options (i.e. the preset options) in Mail > Add Account, it seems to deny you the ability to then edit outgoing mail settings.

The result of choosing to add a mail account using one of the preset options in the above box, is a Server Settings page that appears as per the image in your second post (i.e. there are no settings, and adding an outgoing server from the dropdown menu does not appear to work):

To resolve this issue, I simply removed the account then added it again using Mail > Add Account > Other Mail Account. By selecting "Other Mail Account" rather than using the preset options for AOL (or, in your case, iCloud), Mail allowed me to input the settings manually. As a result, the Server Settings page appeared like this (account info removed for obvious reasons):

Outgoing email from my AOL account works perfectly now. I hope this resolves your problem too if it is still ongoing.

Hello !

Make sure your system is set as online .

if it is offline change to online .

A link is attached , I hope so it might help you this is an example for gmail account : https://discussions.apple.com/thread/7742611

Thanks !

I'm having the same issue as you since a couple of days ago. Were you able to solve it?

Thank you! This solved my problem with several SMTP accounts..

I have been having the same problem. You saved me from the issue. Thank you very much