certificate verification failed for 192.168.x.x: num=20:unable to get local

[Guest]

[enterat]

Hi I'm new to EFW and I need some help sorting out a problem so I can then concentrate on another problem.

In the SMTP proxy log, when ever a clean email is processed the log reads

postfix/smtp[1029]: certificate verification failed for 192.168.x.x: num=20:unable to get local issuer certificate
Apr 30 13:52:12    postfix/smtp[1029]: certificate verification failed for 192.168.x.x: num=27:certificate not trusted
Apr 30 13:52:12    postfix/smtp[1029]: certificate verification failed for 192.168.x.x: num=21:unable to verify the first certificate

What does this mean and how can I sort it?
I have changed my server ip to x it's not just a blank reference.

Thanks in advance.