Thunderbird POP error "Security Exception"

Thunderbird POP error "Security Exception"

When I try to access Zoho mail on Thunderbird with POP I get this ominous warning message:

Add security exception

You are about to override how Thunderbird identifies this site.
LEGITIMATE BANKS, STORES, AND OTHER PUBLIC SITES WILL NOT ASK YOU TO DO THIS.

Server
Location:  pop.zoho.com:995

Certificate Status
This site attempts to identify itself with invalid information
UNKNOWN IDENTITY
The certificate is not trusted because it hasn't been verified as issued by a trusted authority using a secure signature.

"Confirm Security Exception" or "Cancel"
What could be causing this error?  Is there something I can do to fix the problem without compromising the security of my account or my computer?

I have not changed any of my account settings on Thunderbird.  I checked my settings against the settings shown in the Zoho FAQ and I do not see anything that needs to be changed.

Thanks for your help,
Lloyd