Since installing Sequoia on my Mac, my iCloud emails to my google mail contacts are being returned.

Since my recent Sequoia update I am finding that any emails I send to my googlemail contacts are failing and being returned. Trying to decipher the failure noice it would seem that there is a non compliance issue with my emails with googlemail. I know sometimes this can happen if multiple emails are being generated from one address, but mine are just single personal emails.

Can anyone help or have suggestions. Thanks.