To enable us investigating errors or problems you should provide us
with more information.

Helpful would be: (we don't need all, provide as much as possible)

 * the IP, 
 * the Time (including timezone), 
 * the Message-Id, 
 * the Adresses you sent from and to, 
 * maybe some more things that could be related with that problem.

We maintain a system with more than 150000 subscribers which gets
and sends a lot more than 1000000 Mails a day, without that 
information we have no chance to find out what went wrong.