Any ideas how I fix this?
GMAIL Notice from MAIB sent email: "Why is this message in Spam? It has a from address in fwla.com but has failed fwla.com's required tests for authentication."
Also, are you absolutely sure that MiaB sent that mail? It’s common for programs to try to send mail on your behalf.
I know this is a bit old but this error most likely indicates a SPF policy failure, probably a soft fail on the sender SMTP IP for the sending main domain.
Hope this helps.