If you’re using a custom domain, don’t use Mailbox.org, see below:
https://userforum-en.mailbox.org/topic/anti-spoofing-for-custom-domains-spf-dkim-dmarc#comment-1524
If you’re using a custom domain, don’t use Mailbox.org, see below:
https://userforum-en.mailbox.org/topic/anti-spoofing-for-custom-domains-spf-dkim-dmarc#comment-1524
+1 for Migadu. Their basic plan (more than enough for most people) is extremely cheap. No vendor lock in. And their support team is by far, the best I’ve encountered.