Email send problem - "5.7.0 From address is not one of your addresses"

I have seen others have had this problem before but I have not seen detailed steps to actually solve so posting my problem here:

Macbook pro is my primary computer

Also have an iphone and ipad

My main email address is "name"@zoho.com - have had for several years

I use eM Client email software to send and receive email - have used for several years

I recently created a new "myname"@icloud.com email address on my Macbook to serve as a backup and can send and receive email via that address without issue.

HOWEVER I cannot send email (can receive) from my zoho.com account without getting this 5.7.0 error.

"name.icloud.com - 5.7.0 From address is not one of your addresses".

Why is it trying to synch with icloud.com address when I'm sending from Zoho? Perplexing.

I do NOT have a custom email domain nor do I want to create one.

I just want to be able to send email from my Zoho.com account as I have always done in the past.

One "fix" (painful) is for me to abandon my zoho.com email address altogether and fully embrace the icloud address.

But would prefer to continue to use my zoho.com as my main account with the icloud address as a backup.

Can someone here please help? What am I missing?

THANK YOU!