Can't send mail to Icloud account. Says it is full

Open mail app , in inbox right click on iCloud and use get info see the article View email account information in Mail on Mac - Apple Support

Free up storage space for email accounts in Mail on Mac - Apple Support

Does the recipient have sufficient available iCloud storage for the size of the message?

The failing recipient address might be a cc or bcc of the mail message, too. Check the details of the error response for that.

The recipient will need to: Manage your iCloud storage - Apple Support

Whichever iCloud account (presumably yours?) is referenced is full or it’s getting spammed or getting overrun.

(I’m here assuming that you’re not trying to quickly send or forward or transfer a whole lot of email to this address.)

Quota or spam activity or getting overrun, none outside of Apple have any visibility into.

You’re going to need to discuss this with iCloud support.

Contact - Official Apple Support.

Postmaster information for iCloud Mail

This has happened, in a way, with me. Several organisations, government and utilities send me snail mail notifications that they can't send to my iCloud email address. I do actually get the email they are talking about.

It seems to be an issue with them getting a confirmation of the email being sent.

I can use the email account with my IP company. I would however, like to use iCloud.

I am assuming this is the same issue as the op. If not I will repost.

Cheers

Please post the text of the SMTP error message being shown by Gmail, with any sensitive data expurgated.

Contact Apple iCloud Support. They have access to server logs. We do not.