cannot receive emails from gmail at my custom domain

Hello,

I just switched my custom domain to mailbox.org. Everything seems to work fine, I can send emails between users of that domain and to other domains, including gmail. I can also receive emails from other domains (e.g. protonmail, tuta, mailbox.org and my employer's domain), but not from gmail. I don't get any error message, the emails simply don't arrive.

But I can without problems send emails from the same gmail account to my @mailbox.org email.

I have followed the steps in the knowledge base for "SPF, DKIM, and DMARC - How to improve spam reputation and avoid bounces" (https://kb.mailbox.org/en/private/custom-domains/spf-dkim-and-dmarc-how-to-improve-spam-reputation-and-avoid-bounces/) and also checked the entries I made using dig on Linux. I also checked the "domain health" using mxtoolbox and the only thing it found was that the three mailbox.org mailservers are "Blacklisted by UCEPROTECTL2". But that can't really be the reason because then emails to my mailbox.org address would also not arrive.

Before my switch to mailbox.org, my domain could receive emails from gmail just fine.

I guess I am still missing something in the domain configuration, but I am at a loss.

Any hints?

Just in case anybody is wondering: This is the standard plan and I am not in the trial period but paid for it.

twm