iCloud mail not working
iCloud mail not working Message Mail not sent server not found
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See the link Mail - Official Apple Support
Mail server settings for iCloud email clients - Apple Support
Change Outgoing server settings in Mail on Mac - Apple Support
If iCloud Mail isn't working - Apple Support
If you can receive messages but can't send them
Your ISP might be blocking the protocol used to send email (SMTP) across their network. Contact your service provider and ask about SMTP traffic that crosses their network to an external service, such as iCloud. If your service provider blocks SMTP traffic, ask them how you can be exempted from SMTP blocking.
If your ISP isn't blocking SMTP traffic and you still can't send messages, contact Apple Support for help.