[Exim] exim 4 strange error ?!?

I've found in the mainlog this entry:

16nfnK-0000Ct-00 == belindadrieghe@??? R=dnslookup T=remote_smtp defer
(-1): smtp transport process returned non-zero status 0x000f: terminated by
signal 15


Somebody knows which could be the reason of this ?

Thank you,

Raphaël