at least gmail.com has a dmarc policy p=none, so a failing dmarc check is not a reason to reject email. i don't think it's that common for small-scale installs

at least gmail.com has a dmarc policy p=none, so a failing dmarc check is not a reason to reject email. i don't think it's that common for small-scale installs to enforce dmarc policies. there are other signals to use though. plenty of bigfreemail spam is actually sent from their network, they're an interesting target for spammers, and at least some of them put a lot of effort in preventing abuse.