Another possible solution: would be to forward to any other email service that does support POP3 but doesn't have spam filtering. That way, you're not sending email to GMail, it's pulling and filtering. Appreciate that a 3 mailserver setup is odd though, but then so is GMail.

I think it's worth just having a POP3 post in Feature Requests though, purely as a way to allow for any other 3rd party email someone wanted to do this with. I can see privacy advantages in POP3 as well, if one didn't want emails stored on their Cloudron and just locally. I wouldn't like to see POP3 ignored as without advantage, when if privacy is a concern, that could be a legitimate use-case.