Further note:
If you want to check how well this will work before trying it, you can view the source of a few incoming emails:
- Open an incoming email in the web mail app.
- Along the top there are options for replay, forward, delete, then 3 bars menu button, click that.
- Choose 'View source' from the menu.
- Look for 'Authentication-Results'
Below 'Authentication-Results' you see most legit emails say 'spf=pass' while most spam won't.
SPF is a protocol that was introduced to reduce spam, so it works quite well.