Thanks for the answer and the ideas, that helped me, I'm in it and working with it.
But I have a new problem: sending emails via a smart host. I've had various Postfix installations over the last few years, and it's always worked, even with multiple domains and different smart hosts.
With Grommunio I can set whatever I want in the main.cf and associated files. The external Smarthost server refuses to send the message:
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Mai 02 17:08:00 grmail postfix/smtpd[2621]: 3AF6D3C0D29: client=localhost[::1]
Mai 02 17:08:00 grmail postfix/cleanup[2627]: 3AF6D3C0D29: message-id=<gxES.11111111@1111111111.xz>
Mai 02 17:08:01 grmail postfix/qmgr[2143]: 3AF6D3C0D29: from=<user1@aaa.aa>, size=838, nrcpt=1 (queue active)
Mai 02 17:08:01 grmail postfix/smtpd[2621]: disconnect from localhost[::1] ehlo=1 mail=1 rcpt=1 data=1 quit=1 commands=5
Mai 02 17:08:01 grmail postfix/smtp[2629]: 3AF6D3C0D29: to=<extern@bbb.bb>, relay=sslout.df.eu[134.119.18.24]:587, delay=1.4, delays=1/0.01/0.35/0.05, dsn=5.0.0, status=bounced (host sslout.df.eu[134.119.18.24] said: 550 relay not permitted (in reply to RCPT TO command))
Mai 02 17:08:01 grmail postfix/cleanup[2627]: AA26B3C0D3C: message-id=<20240502150801.AA26B3C0D3C@grmail.aaa.aa>
Mai 02 17:08:01 grmail postfix/bounce[2630]: 3AF6D3C0D29: sender non-delivery notification: AA26B3C0D3C
Mai 02 17:08:01 grmail postfix/qmgr[2143]: AA26B3C0D3C: from=<>, size=2798, nrcpt=1 (queue active)
Mai 02 17:08:01 grmail postfix/qmgr[2143]: 3AF6D3C0D29: removed
Mai 02 17:08:01 grmail delivery-queue[926]: remote=[::1] from=<no.envelope.from@invalid> to={user1@aaa.aa>} return OK, queue-id:3
Mai 02 17:08:01 grmail postfix/smtp[2629]: AA26B3C0D3C: to=<user1@aaa.aa>>, relay=::1[::1]:24, delay=0.02, delays=0.02/0/0/0, dsn=2.0.0, status=sent (250 Ok queue-id: 3)
Mai 02 17:08:01 grmail postfix/qmgr[2143]: AA26B3C0D3C: removed
Various instructions for this are known, including here in the forum. Nothing has worked so far.