Hi Paupersumego,
Thanks for reaching out using Apple Support Communities. Here to help.
We understand you are having trouble sending emails to iCloud based email accounts, is that right?
Can you tell us more about what happens when you try to send a message? What is the error message received?
You may wish to contact your email provider or perform further isolation testing. Was the content of the message large enough that file size could be a concern? The following article discusses message size limits for senders and recipients using iCloud email.
Mailbox size and message sending limits in iCloud
How many contacts were affected? Were ICloud.com based addresses the only addresses affected? This next resource is intended for iCloud mail users, but may contain helpful information for you and the intended recipient to troubleshoot the issue.
If iCloud Mail isn't working
Hope this helps! If we can assist further, please reply with additional information and we will happily assist.