Configuring Mailbox.org and Addy.io

I previously used addy.io (used to be called AnonAddy) with my Proton Mail account with no issue and no additional configuration. However, when I moved to Mailbox.org recently, I kept getting bounce-backs with unconfigured (misconfigured?) DKIM, DMARC, and SPF policies, despite having configured them in NameCheap (my host). Ugh. Issue appears to be the policies for my custom domain.

After trying several things, one thing that works is linking Addy not to my custom domain on Mailbox.org, but the default Mailbox.org (xyz@mailbox.org) email that comes with any account. Those DKIM, DMARC, and SPF policies, which are set by Mailbox.org automatically, are intact, and Addy works seamlessly. Just remember to change the sender from xyz@customdomain.com to xyz@mailbox.org when sending an Addy-facilitated reply.