Can’t send out emails

That is only one limit. There are others described in that Apple Support document, so examine all of them.

Apple is sensitive to misuse of iCloud Mail for sending junk mail or what they deem to be "commercial" or non-personal use and they don't divulge the algorithms they use to determine that. If you have reason to believe you are being unjustifiably or inexplicably prevented from using iCloud Mail, and the problem does not resolve itself automatically, the only way to fix it is to contact Apple Support, here: Contact Support.

Be prepared for them to refer you to the same Apple Support document I already provided and discuss it as though you don't already know. You'll need to be patient.

It seems to me that the email service you are using imposes a limit on either the number of email messages that you can send, their size, the number of recipients, or some combination of them.

For iCloud Mail, please read Mailbox size and message sending limits in iCloud - Apple Support.

The answers are in the Apple Support document I sent: Mailbox size and message sending limits in iCloud - Apple Support