Post author here.

I kept subscribing to newsletters and not getting the confirmation emails. Turns out they were in my spam folder, because I forward my email from a custom domain to my gmail account, and that means SPF checks always fail.

I found for my own newsletter, sending from my verified domain solved the problem for forwarded emails, despite the fact that my marketing provider (ConvertKit) says not to send from your own domain until you're sending to 5000+ subscribers. I did a lot of research on how SPF, DKIM and DMARC work to understand why things were breaking and why marketing email providers seem to be all over the map on whether they suggest you use a verified domain and when.

Figured it was worth sharing, let me know what you think.