Hello taueva.
Thank you for using Apple Support Communities and for sharing these details with us. We'd like to do our best to assist you in any way we can. To confirm, is this specific issue only occurring with your iCloud email account or with all other email accounts you have set up on your Mac? If it only occurs with your iCloud account, does it occur with all emails you send our or was this a one time occurrence? If you visit the iCloud website, does the specific email you sent out show up under "Sent"?
iCloud - Apple
Sign in with your Apple ID & password. Once you sign in, select Mail and click on Sent. If it does populate there, you'll want to attempt some of the troubleshooting steps listed in the resource below:
If iCloud Mail isn't working - Apple Support
Feel free to keep us posted on your results. Cheers.