Forward emails => from adress is not one of your adresses

My gmail-emails are forwarded to my icloud account, so I’ve gathered my emails from two accounts in one account.
And in em client I only check the icloud-account, but when I want to forward an email that was originally sent to my gmail-account the from-field ist automatically filled with my gmail-account which is not set up in em client which causes em client to not sent my message and showing the error “from adress is not one of your adresses”

I always have to change the from-field to my icloud-account manually.

Well, is there a setting oder a fix for this issue?