For several days, I could not send email from Apple Mail using the iCloud SMTP server. I have done the following: Signed out from Apple ID - from System Preferences/Apple ID/Overview, there is a sign out button. This will sign you out from all apps using your apple ID e.g. Mail, App Store, iTunes, TV, etc. Rebooted the mac, signed back to Apple ID. In the iCloud section, check the services I want to enable e.g. Mail. Mail would re-add your iCloud IMAP and SMTP settings. Unfortunately, email sending would still not work. An error dialog would pop up with an error message: <unknown[xxx.xx.xxx.xxx]>: Client host rejected: Access denied Using Mail's connection doctor showed that everything was connected, even the iCloud SMTP server. I removed the iCloud SMTP server and tried to recreate it. When I used my iCloud password, the SMTP went offline. Connection doctor says it could not log in to the SMTP server. After several days of Googling, I found this Apple support article: Using app-specific passwords Following the directions, I inspected my Apple ID account page and found that I had 1 generated password for this mac, specifically for the SMTP server. I deleted that and generated a new password. I used the new generated password for the SMTP password and it went online. I could now send email from my iCloud accounts again.