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Hello,

I get the following error message in my logs /var/log/mail.err How do I fix it?

it means that Dovecot is trying to bind to an IPv6 interface, but your system doesn't support IPv6.
The solution is to tell Dovecot to only listen on an IPv4 interface. You can do this by adding the following to the very top of your /etc/dovecot.conf file
instead of