Hey deworama!
Can you say the amount of free iCloud Storage you have?
It sounds like you’re aware of Apple’s article regarding Mail Drop Limits:
https://support.apple.com/en-us/HT203093
In which if you used the quota, only after a month of one large attachment dropping off will free it up, although 1TB is a lot of room.
Thats a good question though, I don’t think there is a public interface showing Mail Drop quota though, but knowing your free and used iCloud storage may help.
Likewise, are you not able to Mail Drop to ALL recipients? Or just some people? It’s possible THEIR Mail quota has been reached or you are being blocked.
Likewise, if you send via Mail Drop from different locations and devices, do you get the same error message? (Such as from iCloud.com, or Apple Mail on this or another device, or even on another network?)?
Those maybe good tests to find out a cause, but as for finding Mail Drop quota used, can’t say, but I’d assume it would count towards your iCloud Mail Storage, when you check your iCloud Storage:
https://support.apple.com/guide/icloud/check-your-storage-usage-mm6e5a441fc7/icloud
Hope something here helps!