Try Vineyardsoft's web site, they are the creators of Event Manager.
Look under support:
I found this:
General SMTP Mail Failure, check setup = 6000
Solution:
This usually means that the SMTP server is shutdown for some reason or the server that it's running on cannot be located on the network.
Things to try:
On the KnowledgeSync server
Run
Cmd.exe
Try pinging the SMTP server:
ping yourservername
Make note of the IP address returned.
If that fails, check the network for problems.
If that works, configure KnowledgeSync to use the IP address returned by PING of the SMTP server rather than the SMTP server name.
There could be a problem with the DNS server on your network.
Check also to make sure that a firewall was not recently installed that may be blocking the port=25.
General SMTP Mail Failure check setup = 6000 ]
AND Also this..........
-2147215504 General SMTP Mail Failure, check setup = 6000
Solution:
On the Email Tab for the Event, check and verify that the Email Account to Send From is not blank.
[ 2147215504 General SMTP Mail Failure check setup = 6000 ]