Can't send mails due to error message: timestamp check failed

Can't send mails due to error message: timestamp check failed

Hello,

I have the mail add-on for your CRM which always worked well. 

Since a couple of days I get this error message: 

sun.security.validator.ValidatorException: PKIX path validation failed: 

java.security.cert.CertPathValidatorException: timestamp check failed

What does this mean? Are my mails still sent out correctly or not?

thanks,

kind regards,

Lars