[Exim] 421 Lost incoming connection

What causes this?

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