Try the following fixes if you are having the issue of 'iCloud Email not sending:
Check your Internet Connection
Restart Mail App
Check iCloud Status
Check Outgoing Mail Server Settings
Update macOS

Facing trouble fixing the 'iCloud Email not sending' issue? We have showcased this video to help you resolve the same issue with ease. Follow these 4 quick solutions.
Solution 1: Check Network Connection (00:00:10- 00:00:58)
Solution 2: Force Restart (00:01:02- 00:01:32)
Solution 3: Ensure that Mail is Turned On (00:01:37- 00:01:54)
Solution 4: Delete & Reinstall the Mail App (00:02:00- 00:02:49)
Watch the video until the end to fix the 'iCloud Email not sending' issue.
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