Emacs 29.1 MacOS and smtp configuration for mu4e

Question

I try to configure the SMTP part in the ``mu4e config and have configured:

...
 (setq message-send-mail-function 'smtpmail-send-it
      smtpmail-stream-type         'starttls
      smtpmail-default-smtp-server "mx2fd2.netcup.net"
      smtpmail-smtp-server         "mx2fd2.netcup.net"
      smtpmail-smtp-service        587
      smtpmail-local-domain        "my.local.name.de"
  )
...

for a netcup account and

~/authinfo

with several machine access data. On a Linux system (Devuan dedalus Emacs 28.2) the E-Mails are sent properly, while on a MacOS system (BigSur Macports Emacs 29.1) with the same configuration, the SMTP process signals

smtpmail-send-it: Sending failed: 554 5.7.1 <IP6:PORT>: Client host rejected: Access denied

I understand that the mail client and server MX won't talk to each other, but what differs in the Apple vs. Linux setup?