email not reaching my inbox

This is the mail system at host relay.mailchannels.net.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

The mail system

<xxxx@mailbox.com>: host mailbox.com[185.230.63.107] refused to talk to
me: 421 Downstream server error

Reporting-MTA: dns; relay.mailchannels.net
X-Postfix-Queue-ID: 4A3727E2942
X-Postfix-Sender: rfc822;xxxx@xxxx.nl
Arrival-Date: Tue, 09 Jun 2026 09:31:06 +0000 (UTC)

Final-Recipient: rfc822; xxxxx@mailbox.com
Original-Recipient: rfc822 xxxxx@mailbox.com
Action: failed
Status: 4.0.0
Remote-MTA: dns; mailbox.com
Diagnostic-Code: smtp; 421 Downstream server error

EDIT: We edited the mail-addresses, so that they are not public. Your mailbox team.