Mail forwarding broken

As per this article : Mailbox size and message sending limits in iCloud - Apple Support

Limits on sending messages

There are several safeguards in place to make sure that only iCloud members can send messages using iCloud. One of these safeguards includes reasonable limits on:

• The total number of messages you can send each day (1000 messages)
• The number of recipients you can message each day (1000 recipients)
• The maximum number of recipients per message (500 recipients)
• The size of incoming and outgoing messages (20MB, up to 5GB with Mail Drop turned on)
• When you exceed message sending limits, you might see the following alert messages depending on how you view iCloud Mail:
• On your Mac: Cannot send message using the server iCloud.
• On iCloud webmail: 550 5.7.1 membername@icloud.com: You have exceeded your daily sending limit. Please try again later.

You can verify it by opening in your I cloud mail , right click on it and click on get account info .

By , clicking on get Account info another window appears .

You can see the size of mail box : currently in use and the percentage slider is more than 50 % , to resolve it ,select the sent box & click on show messages , you have to delete unwanted messages .

The above screen shot is just for reference as it is not showing the current storage percentage as I am using very less I cloud mail in my computer .

Contact iCloud Support. You may have to try twice.

iCloud Support

Have you tried signing out of iCloud and back in? If you stop forwarding, do you receive the e-mails at your iCloud account?

Open iCloud mail , Click on window > connection doctor , is SMTP ( simple mail transfer protocol showing red dot ) for I cloud .

You are welcome. If you would post what was done, it might help others.