Mail will not send - OS Sierra

If you have a Virtual Private Network (VPN) in operation, Mac High Sierra gets confused. I shut off my VPN and mail is working again. I have the same problem with other outgoing connections, like Daz3D Studio installer, and the App Store. They are also working again. I think this an engineering bug, a bug that will make life difficult for users who communicate with their workplace via VPN.

If anyone is still having this issue, I share your pain. It cropped up again for me today. Maybe I have a fix for you.

The following relates to my being on MAC OS Sierra 10.12.3

My hunch is this issue arises when switching between the "From" email address drop-down list when creating a new email and or in conjunction with switching on or off mail or any mail accounts within the same session of using Mail. I don't know why but it all seems correlated that when I use multiple accounts to compose emails and or happen to switch off an account or change the from address it's triggering a bug with the server selection for each account. It's possible my university's mail server settings for my uni account is interfering with my other accounts server settings each year I update it. I really don't know.

Anyway, here's my solution. I troubleshot the SMTP server countless times and did a few other activities short of getting under the hood on the OS. I prefer not to do that.

Fortunately and finally I found a simple solution.

1. I first disabled/ unchecked the iCloud mailbox via, settings>internet accounts>iCloud [Tab]
2. I then went to mail and selected Mail>Preferences>Accounts tab
3. I made sure I deleted any SMTP servers I had created/ added in previous attempts to resolve the issue so that nothing was left in the "Server Settings" [Tab].
4. I then went to the "account information" [Tab] where it says "Enable this account" and I unticked the checkboxes for each mail account I have. Everything was off and I could see nothing in mail folders at the main interface of the app.
5. I then quit mail and waited about a minute.
6. I went back to settings>internet accounts>iCloud [Tab] and checked the box to enable the iCloud mail.
7. I then opened Mail and checked the "Enable this account" boxes for each email account.

I did not touch SMTP, just switched off and on the accounts and now its resolved.

Hope it works for you too!

My father just ran into this when upgrading his iMac to Sierra today. I fixed it by going to the Mail->Preferences and Accounts tab, selecting the iCloud account and then un-checking the 'Enable this account' checkbox. I don't recall if I closed the Preferences window, but I re-enabled the account and then he could send email again.

Thank you, Nicodemeus. I have been searching for days for a solution. This worked very well for me. I do not use iCloud for email at all, but I disabled the iCloud Internet account anyway, along with every other of my multiple email accounts. Turned off Mail, then shut down my computer.

I believe I enabled them again with Mail/Accounts/etc. which is the same, I think, as System Preference/Internet Accounts. BUT, I did not try to assign an SMTP or go into advanced settings.

Then, when I tried to send an email with a specific email address, MAIL set up the SMTP for me for that address, and I indicated to use it all the time.

It worked!

Interestingly, when I go to Mail/Preferences/Accounts and Server Settings, Outgoing Mail, and use the drop down menu to go to "edit SMTP list", there is nothing in the box! I guess I don't care, since before there were so many that I didn't want and could not delete, and it seems to be working well for now.

Thanks very much. Any other comments on the above would be appreciated.

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Have you used the Connection Doctor option offered in the dialog box you posted?

Try the Connection Doctor, this time check the Log Connection Activity checkbox, Check again, then Show logs.

Interesting. The second log dealing with the SMTP outgoing mailserver shows it trying to connect to Port Zero which is never going work. In your settings, does it still show port 587?

If it was me at this point, I'd be deleting the mail account, rebooting in safe mode, rebooting and setting it up again from scratch.

So, just to confirm, when you go to Mail > Preferences > Accounts > Server Settings, you have no outgoing mail server details listed?

There's the problem, the outgoing SMTP server port is set to Zero and the others details are missing.

What happens if you use the + sign on that screen to add the outgoing SMTP mail server and all its settings?