I have a vanity domain, and used to be able to use GMail to send email as that domain [1], and forward received mail from that domain back to GMail.
But with SPF, DKIM, and DMARC (or something), this has broken and such received email gets marked as spam and/or phishing.
I don't know how to fix the forwarding/receiving flow [2], because GMail will see the message coming from an arbitrary source, but being forwarded by an intermediate (my hosting provider's forwarding email server) which will fail its integrity checks.
I have not been able to understand how to solve this. Clearly, forwarding servers aren't a well-regarded use-case in this new era of verified email flow.
Also, I'm not just talking about GMail. I used to do forwarding for my family who used a variety of providers, so I can't just switch to GMail (via Google Domains) for my entire domain.
[1] GMail still offers that feature, under Settings->Accounts and Import->"Send mail as"
[2] the sending flow is easy: just add gmail's SPF to my own domain's