I'm creating a flow and the last step should send an email (am using SendGrid)

The problem is that in that last step, it won't let me add external email addresses in the "To:" field:
Here is the screenshot with the Flow checker error message I get. Although I can write down "agurovij@gmail.com" , the flow checker doesn't accepts it. ( NOTE: In the "From:" field I added my Tenant email alexgurovich@indidev.cl and it was recognized with no issue as Alex Gurovich. If in the "To:" field instead of agurovij@gmail.com I wrote down alexgurovich@indidev.cl , it would work).
So, it seems there is a problem recognizing emails outside of my Tenant. Why?
