Are you hosting a custom domain on mailbox.org or do you have a @mailbox.org address? If it's the latter, mailbox is then not really an email relay. Generally, email relays are able to forward all addresses of a domain i.e anything@cloudrondomain.tld.

thanks for the explanations. I have already contacted digital ocean requesting the release of port 25. at first, strangely, they claimed that there is no output block on port 25 (although I did the test with the command 'telnet' in the cli).