Catalina iCloud emails sending via Gmail instead

I have a 10.15.5 2017 iMac (Catalina) and have just moved got my emails set up in Apple mail.

I made sure in the account settings for each of my accounts that the correct SMTP settings are set. I have 3 accounts:

1 - iCloud @iCloud.com

2 - Gmail @gmail.com

3 - Personal @XXX.id.au

When I send using the Gmail or Personal accounts everything is fine. However, when I go to send from the iCloud account it shows up in the Gmail sent folder and the recipient shows that it was sent from the Gmail account as well.

As I mentioned, the iCloud SMTP selection is set to iCloud. The Gmail one is set to Gmail and Personal is set to the custom SMTP settings in the SMTP list for that email host.

I'm making sure that I'm selecting the correct account to send from when I send the emails.

There is no prompt or alert indicating that it couldn't send and was trying another SMTP server.

If I delete/deactivate the Gmail account, then the emails go through as expected.

It feels like there's some kind of USE ONLY THIS SERVER option that's unselected for the iCloud account, but I have no idea where to change this. I want it to use the iCloud SMTP server exclusively but I don't seem to be able to set this.

Can I get some help here please? Thank you.

Tim

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