You're absolutely right, ending up in spam is a real pain. I've run into the same issue myself with custom domains; even when everything is set up correctly, delivery can still be unreliable.
This is more about reputation than “custom domains”. I’ve had <realname>.com registered since 1996. Hosted at free Google Workspace account for 15+ years. They added SPF and then DKIM/DMARC as those thigs evolved. Never had spam reports in all that time; delivery is better than $dayjob’s 30-year-old domain.
I set all that stuff up for mine, and used a reputable mail server (FastMail), and even went through steps particular to Microsoft and Google. Google started accepting my emails, but my employer's Microsoft Exchange system kept sending my stuff to spam. How many other people would just not receive my stuff? It's impossible for me to know. If you want to reach out to strangers, any false positives will hurt you.
tatersolid|2 months ago
Reputation is everything in email delivery.
wakawaka28|2 months ago