Understanding SPF, DKIM, and DMARC: A Simple Guide

If you do SRS correctly it will not pass DMARC alignment for your domain but it will pass plain SPF which does not have the DMARC alignment check and is sometimes checked independently from DMARC. If the sender included valid DKIM it should pass DMARC for the sender's domain as long as you don't alter the signed parts of the message (unless possibly if they do something annoying like sign the absense of X-Forwarded-To). Google also wants you to use ARC, add X-Forwarded-{To, For} headers, avoid forwarding spam, and use a different IP address or domain for forwarding vs sending mail from your domain. Some email providers let you indicate that you trust particular ARC forwarders but I don't think Google uses it that way.

https://support.google.com/mail/answer/175365?hl=en

I don't know why Google want to force forwarders to do spam filtering.

You're probably right about the terminology, sorry. My problem is that a lot of legitimate senders have failing (or soft-failing) sender setups, so I can't have my forwarder just drop all that (I'm not even sure my registrar-provided forwarder has that option).

Another option would be to have another inbox on the domain and have Gmail fetch with POP/IMAP, but many domain registrars don't have that service. Or is that what most people do?

For receiving custom domain email long term I think the usual thing would be to have the chosen email provider manage all email for the domain (setting up MX records) and not forward long term. That would require a paid account with Google it looks like, but at the point you are paying for your domain that seems reasonable (I'm not sure how much they charge). The fetch options are usually either time limited or expensive from what I've seen. I wouldn't be surprised if registrars don't forward correctly according to Google's instructions, they just want the least expensive thing that lets them advertise email as a feature, but maybe worth complaining anyway if they don't.

For personal accounts I don't think you can convince Google to accept mail from incorrectly configured senders, you would need to use a different service if you must receive it. For business accounts they are less strict, although Google still randomly drops mail and I wouldn't personally use them for that reason and others (I think they have good suggestions for how mail should be set up, though). I'm not trying to sell you anything and haven't even tried them yet (and have some questions I haven't asked yet) but purelymail.com is one that seems promising to me in general and is inexpensive with no extra fee for custom domains (if you aren't attached to the way Google handles mail; I'm guessing the archive search and general interface will be more clunky). I'm guessing you can configure SpamAssasin to not strictly reject mail from incorrectly configured senders as long as they aren't on spam lists (they say they reject those immediately), but I'm not sure.

Thinking about it more I'm guessing Google requires forwarders to check spam due to not wanting customers to configure trusted ARC forwarders like Microsoft does and so not forwarding spam makes it easier to convince them the forwarder isn't a spammer trying to blame others for the spam (they might also compare forwarded spam with what they receive directly). If a forwarder uses ARC and can convince them they aren't spamming then Google should trust their SPF assessment if you just mean senders who only have SPF configured. But if you want to receive email from old timers who insist that email should have no way to detect spam there may be no way to do that with Google (or a number of other providers).