using external relay error connection died #4.4.2

i configured ISPConfig for using external relay ( in my case serversmtp.com ) but if i try send mail from diffrent domain but stay in same server not recevied mail and on dashboard provider relay i see connection died #4.4.2.
But this issue only from few day
can help me ?

 

The number alone, tells us almost nothing. Please provide the full error message.

 

Unfortunately, I can't get the details from the supplier. The only thing I managed to get was the log, which says they queue them, but not the exact reason. They keep telling me that since the two domains are internal to the same server, I have to whitelist them. But I never did, and yet it worked before.