Hello,
I've numbered accounts to simplify the question.
1) Account which I would like to be the email sender
2) Administration account
3, 4) generic users
I've created a flow with a powerapp trigger which send an e-mail:

The sender of the e-mail must be Account 1 "support@contoso.com" which is different from both the flow author (2) and the powerapp user.
I've created a connection for user 1 and given that to the "send email as" task on the flow.
If I trigger the flow from the powerapp it goes smoothly but if another user does it, I receive a "Forbidden" error.
Flow runs:

This is the error message:

I would like to highlight this from the error message:
"code": "ErrorSendAsDenied"
and the error details roughly tranlsates to:
You are not authorized to send an email as the email account sender specified
To trigger the event I've simply put a button on a power app which runs the flow onSelect, The successful runs are from account 2 and the errors are from a generic account 4.
Notice that in the browser with account 2 I was also directly logged in to account 1 so that might be why account 2 can send emails for account 1
How can I solve this?
Thanks