**Update 21 Oct 2015: Apple released OS X 10.11.1 which claims to have fixed this problem**
After upgrading many Macs to OS X 10.11 El Capitan, I’m finding a bug that seems to be quite consistent. Virtually everyone who uses an email address ending in @me.com is suddenly unable to send messages using the built-in Mac Mail program. Emails still come in but the outgoing messages don’t go. Fortunately it’s a very quick and easy fix.
- Disable and re-enable your iCloud email.
- Go to the Apple menu > System Preferences > iCloud (3rd row).
- Click the checkbox next to Mail to disable it.
- After waiting for the checkmark to disappear, click the box to turn Mail back on.
- You can close the System Preferences window.
Set your outgoing email server.
- Open Mail.
- Click the “Mail” menu and choose “Preferences”.
- Switch to the “Accounts” tab.
- Choose your iCloud account from the left column.
- Look for “Outgoing Mail Server” on the right and switch it to “iCloud”.
- Close the Preferences window to return to Mail.
That should do it! If you have a lot of email in your iCloud account or a slow connection you may need to wait some time for your email to re-download, so don’t worry if your email inbox is empty for a few minutes.