Hey Gardenweed - thanks for the reply!
I can't set any SMTP settings there. All I see is this:
https://www.evernote.com/l/AosFoGWEq...WdoWW6kIIraPYA
Pressing "re-authenticate" just brings up the standard Google dialog box, where I can choose to authenticate with the main account:
https://www.evernote.com/l/AouA0-xuq...zeRu5gdSBCSSRM
Probably there's a bug with this on FastMail's side - since re-authenticating works, but then email sending gets a "username/password is incorrect".
At the point of email sending, I think FastMail tries to authenticate using the IDENTITY email set in FastMail, instead of the Google ACCOUNT email needed for authentication.
And there is no way to set this manually. :/