Check Your Side
Check the Outbox and Sent Folder:
Look in your Outbox for the message. If it's there, it hasn't been sent yet. This usually points to a connection or setup issue.
Look in your Sent folder. If it is there, the email left your account, meaning the problem is likely further down the line (recipient's server, spam filter, etc.)
Verify the Recipient's Email Address:
It's a simple step, but make sure the email address is spelled exactly right. An "undelivered mail returned" message (a "bounce-back") is a strong sign of a bad address
Check Your Internet Connection:
Make sure you have a stable Wi-Fi or cellular connection. Try sending an email while connected to a different network (e.g., switch from Wi-Fi to cellular data)
Check Your iCloud Storage:
If your iCloud storage is full, it can prevent you from sending and receiving new emails. Go to Settings > [Your Name] > iCloud to check your usage. If it's near the limit, try freeing up space
Check Apple's System Status:
Occasionally, Apple's services have temporary outages. Search for "Apple System Status" to check if iCloud Mail is currently experiencing issues (it should have a green indicator)
Message Size:
If you included large attachments, the total size may have exceeded the limit allowed by your email provider or the recipient's. Try sending a simple, text-only test email. If the test works, the original message was too big.
Check the Recipient's Side
Even if the email is in your Sent folder, it may have been intercepted by the recipient's server.
Ask the Recipient to Check their Spam/Junk Folder:
The email may have been filtered as spam. Ask them to look there and, if they find it, mark it as "Not Junk" or "Not Spam" to prevent future issues.
Ask the Recipient to Check for Blocks/Rules:
If the recipient has accidentally blocked your email address or set up a mail rule that automatically filters or deletes your messages, they won't receive them.
Next Steps
If the above checks don't solve the issue:
Try sending an email from the iCloud website: Go to iCloud.com/mail in a web browser and try sending an email from there. If this works, the problem is likely with the Mail app settings on your specific device.
Restart your device: A simple restart can fix temporary glitches.