Can’t send email
Since upgrading to High Sierra, I’ve been unable to send messages from Mail to any account apart from me.com. Mail reports that “trying to log into this SMTP account failed,” although everything is set as it was in Sierra.
This is an old problem that happens when an update corrupts Mail’s list of SMTP servers. To fix it, you’ll need a copy of the correct details for each of the services you want to use, because you need to delete existing entries and then make new ones.
Open Mail’s preferences and click the Accounts item, select an account on the left, then click the Server Settings tab. This displays a single pop-up menu, with that account’s outgoing mail server listed.
Click that menu, and select Edit SMTP Server List at the bottom. Select each of the SMTP servers in the list at the top in turn, and click the – (minus) button to delete each one until the list is empty.
Now click the + button to add back each of your SMTP servers one at a time, using the details you recorded. Then click OK, and step through each of your accounts in turn, setting its SMTP server from your new list.
Finally, quit Mail, just to ensure everything is saved correctly. When you open Mail again, you should find that sending messages works normally again.
If Mail stops sending to SMTP servers that seem to be properly configured, delete them all and add them afresh.