mwilliams thanks for your answer. I have checked the database settings in the 3 files. They are correct (dbname, password and user). I also added a new domain and rebooted. Still no change: no messages arrive in the inbox.
[2021-09-23 16:10:38.166840]: 28AE01F346: client=mail-gv0che01on2048.outbound.protection.outlook.com[40.107.23.48]
[2021-09-23 16:10:38.197304]: 28AE01F346: message-id=ZR0P278MB0798B0D31F123980C173E4809AA39@ZR0P278MB0798.CHEP278.PROD.OUTLOOK.COM
[2021-09-23 16:10:38.884993]: 28AE01F346: from=X@Y.com, size=25111, nrcpt=1 (queue active)
[2021-09-23 16:10:38.901487]: disconnect from mail-gv0che01on2048.outbound.protection.outlook.com[40.107.23.48] ehlo=2 starttls=1 mail=1 rcpt=1 bdat=1 quit=1 commands=7
[2021-09-23 16:10:38.938930]: 28AE01F346: to=testing@Y.net, relay=::1[::1]:24, delay=0.78, delays=0.73/0.03/0/0.02, dsn=2.0.0, status=sent (250 Ok queue-id: 1)
[2021-09-23 16:10:38.939471]: 28AE01F346: removed