Sending mail in Ubuntu - Service unavailable [closed]

I know there is a lot to find about this, but nothing I read so far seems to be working.

I run Ubuntu with nginx installed and running.

Everytime I want to send a mail I get the error: dsn=5.0.0, stat=Service unavailable

The code:

echo "Hello world" | mail -s "Test" manu@hawc.be

The complete error:

Jul  1 12:21:49 localhost sendmail[8875]: 461ALnoU008875: from=administrator, size=136, class=0, nrcpts=1, msgid=<202407011021.461ALnoU008875@localhost.localdomain>, relay=administrator@localhostJul  1 12:21:49 localhost sm-mta[8876]: 461ALnKi008876: from=<administrator@localhost.localdomain>, size=436, class=0, nrcpts=1, msgid=<202407011021.461ALnoU008875@localhost.localdomain>, proto=ESMTP, daemon=MTA-v4, relay=localhost.localdomain [127.0.0.1]Jul  1 12:21:49 localhost sendmail[8875]: 461ALnoU008875: to=manu@hawc.be, ctladdr=administrator (1001/1001), delay=00:00:00, xdelay=00:00:00, mailer=relay, pri=30136, relay=[127.0.0.1] [127.0.0.1], dsn=2.0.0, stat=Sent (461ALnKi008876 Message accepted for delivery)Jul  1 12:21:50 localhost sm-mta[8878]: STARTTLS=client, relay=mx2.pub.mailpod9-cph3.one.com., version=TLSv1.3, verify=FAIL, cipher=TLS_AES_256_GCM_SHA384, bits=256/256Jul  1 12:21:50 localhost sm-mta[8878]: 461ALnKi008876: to=<manu@hawc.be>, ctladdr=<administrator@localhost.localdomain> (1001/1001), delay=00:00:01, xdelay=00:00:01, mailer=esmtp, pri=120436, relay=mx2.pub.mailpod9-cph3.one.com. [185.164.14.119], dsn=5.0.0, stat=Service unavailableJul  1 12:21:50 localhost sm-mta[8878]: 461ALnKi008876: 461ALoKi008878: DSN: Service unavailableJul  1 12:21:50 localhost sm-mta[8878]: 461ALoKi008878: to=<administrator@localhost.localdomain>, delay=00:00:00, xdelay=00:00:00, mailer=local, pri=30000, dsn=2.0.0, stat=Sent

I was trying to send a mail