why this bounce back comes ?

An error was detected while processing a file of BSMTP input.
> The error message was:
>
> 421 Lost incoming connection
>
> The SMTP transaction started in line 0.
> The error was detected in line 3.
> 0 previous messages were successfully processed.
> The rest of the batch was abandoned.
> 421 Lost incoming connection
> Transaction started in line 0
> Error detected in line 3

Thanks.