Mail warning: hostname verfication failed

Hello,

In my log I see a lot of warnings like this:

Code:

Nov 4 11:42:28 backupserver postfix/smtpd[22340]: warning: 213.56.159.12: hostname mail.ch-lannemezan.fr verification failed: Name or service not known

What does it mean? Are those mails sent or did it fail? How to avoid these warnings?

Thanks

 

Do you have valid nameservers in /etc/resolv.conf?

 

I do,

my resolv.conf:

Code:

nameserver 8.8.8.8
nameserver 8.8.4.4

 

Is this one of your servers or a remote system?

 

it's a remote system. I think it's one of the emails that were sent... and this warning was present after sending the mail

EDIT: i think i found the problem. It's probably some spam-servers which try to send mail through my server ip. I've looked up some recent warning hostnames and found that most of them are actual spam IP's.
Although I have to say that it's a weird error for something like this, but ok.