iCloud Email bouncing back when sent from new Macbook

Good morning RDTokyo!

It sounds like you’re having some issues sending mail using iCloud on your MacBook, even though you’re able to send mail successfully on your iPhone. I’ll be happy to help out with this.

Take a look at the steps here to see if you can resolve the issue where mail won’t send to some recipients on your MacBook:

If your recipient doesn't receive your message

1. If an “unknown address” or “undelivered mail returned” message appears in your Inbox after you send a message, go to your Sent mailbox and make sure that the recipient's email address is correct.
2. If you're using macOS Mail or accessing your iCloud Mail using Safari or another supported web browser, check your Drafts, Sent, and Outbox mailboxes:
   • If you see an unsent copy of the message in Drafts, try to send it again.
   • If you see the message in your Outbox, make sure that you're connected to the Internet and use the steps above to select the iCloud SMTP server.
3. Ask the recipient to check their Junk mail folders or mailboxes.
4. If your recipient still doesn't receive your message, but your message was sent without any alerts or “undelivered mail” messages, your message might have been blocked or filtered by the iCloud Mail servers or the recipient's mail server. To find out, contact iCloud Support. You should also ask your recipient to contact their email administrator.

If you can receive messages but can't send them

Your Internet service provider 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 service provider isn't blocking SMTP traffic and you still can't send messages, contact Apple Support for help.

Get help using iCloud Mail - Apple Support

Cheers!