[exim] Which exim4 command line option produces an id field …

Hello,

In mainlog exim4 files, I can see both types of lines:

2008-03-12 08:53:28 1JZLmC-0000E7-0A <= root@??? U=root P=local S=43802
id=Asterisk-0-123413860-4174-2662@ipbx-bs-60200

2008-03-21 08:51:25 1Jcc29-0000C2-HP <= root@??? U=root P=local S=343


Which command line option generates the inclusion of this "id=" field ?
I'm using Debian etch.

Regards