Hello,
You can replicate signing in with the user's details i've provided. 
-- 
Signing with user's details means with stackideas account or Danny Mills account? 
 and trying to send me an inbox message
-- May I know which account you are using?
I have tested with both side:
1. Stackideas send a message to Danny Mills
2. Danny Mills send a message to Stackideas.
Both work just fine. Please advise.