> > @ IN MX mail.customer.com.
> > IN MX 2nd-mail.isp.com.
> > www IN A 192.168.1.1
> > mail IN A 192.168.1.2
>
> The MX records are wrong which is why the mta's are sending to the A record.
>
> it should be :
> @ IN MX 0 mail.customer.com.
> IN MX 1 2nd-mail.isp.com.
>
> You MUST not forget priorities in the mx records
Ah sh*t... no, they are there in the actual zone-file but I forgot
them here in the example :-)
Thanks for the hint!
Sascha