iCloud: If you have trouble sending email
If you get an error when you try to send email
If you recently changed your iCloud account password, and you use an email app to send iCloud email (instead of using a browser), make sure to update your account information in the email app. Also, if you use the Mail app on a Mac, open iCloud preferences, then enter your new password.
If you use an email app to send your iCloud email, make sure the app’s SMTP (outgoing) server is smtp.mail.me.com or an address provided by your Internet service provider (ISP). To find out your ISP’s SMTP server address, read the information you received when you signed up for your account, visit your ISP’s website or call its support line, or look at the settings for another email account that uses your ISP’s server.
If you use smtp.me.com for your SMTP server, make sure you turn on SMTP authentication in your email app using the Apple ID name and password you used to set up iCloud, and make sure the email address in the From field of your email message matches your iCloud account name. If the trouble continues, have your email app use your ISP’s SMTP server instead.