logs to find notification issues

Where do I find logs to help identify why I'm not getting notification emails any more?

what release and version are you using? (for example Premium Edition version 9.0.15)

Professional Edition - version 7.5.6....I found the logs, but they don't tell me anything. How do I debug / verify messages are going out to eliminate servercheck as a problem?

7.5.6 is a 4 year old version - it's one before my time - maybe another forum member can assist