cannot send email, but receive email fine

Hello,

I cannot send emails from my MacAir, despite manually checking settings and re-entering my password. I am getting message that my iCloud (XX@icloud.com) sender was rejected. The error code provided was AUP#CXSNDR, which apparently means 'Your email failed authentication checks against your sending domain's SPF, DomainKeys, or DKIM policy." How do I fix this??

I saw a great post from early this year with directions that I followed today without success: "There is a bug in the macmail software. you have to change the outgoing mail server settings to manual, and the make the settings to 465, outgoing server requires authentication. and password. Make sure you erase the dots currently in the password field and then re-enter the password. I used to beta test for Apple, and I know that bug well. I actually reported it when they went from 10.4 to 10.5, a decade ago, and they STILL haven't fixed it. Also, ensure, prior to doing all this, delete everything sitting in the outbox."

Thanks!!