[CentOS] Postfix problem with DNS

Today I have changed my DSL modem and problems with postfix begun. It receives mail, but cannot send any. In logs I can read

Jun 22 19:39:53 server postfix/smtp[5040]: 9A51711FF85: to=address@somedomain.com, relay=none, delay=12840, status=deferred (Host or domain name not found. Name service error for name=somedomain.com type=MX: Host not found, try again)

My resolv.conf is proper. I can ping any server, but postfix cannot see them. What can I do ? Postfix 2.1.5

Best Regards Kuba

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