[till]

There are no changes nescessary in the mail setup when the IP changes. You have to change just the webserver settings.

Not sure why you have this lin:

-o mynetworks=xxx.xxx.xxx.xxx (<-- changed this one to the new one)

as it is not from ISPConfig and may not be there as this line prevents amavisd to work correctly. The correct amavisd Configuration is:

127.0.0.1:10025 inet n - - - - smtpd
        -o content_filter=
        -o local_recipient_maps=
        -o relay_recipient_maps=
        -o smtpd_restriction_classes=
        -o smtpd_client_restrictions=
        -o smtpd_helo_restrictions=
        -o smtpd_sender_restrictions=
        -o smtpd_recipient_restrictions=permit_mynetworks,reject
        -o mynetworks=127.0.0.0/8
        -o strict_rfc821_envelopes=yes
        -o receive_override_options=no_unknown_recipient_checks,no_header_body_checks
        -o smtpd_bind_address=127.0.0.1