[exim] virtual user error

Can someone please explain the following message in the exim_mainlog
file to me?

Also, how to fix :)

2005-09-08 14:01:06 1EDPxW-0005fi-9G == mattpike@???
<MattPike@???> R=virtual_sa_user
T=virtual_sa_userdelivery defer (-24): Transport filter process
failed (127): unable to execute command
2005-09-08 14:01:06 1EDQAb-0006LX-My <MattPike@???>:
virtual_sa_userdelivery transport output: An error was detected while
processing a file of BSMTP input.
2005-09-08 14:01:06 1EDQAb-0006LX-My == mattpike@???
<MattPike@???> R=virtual_sa_user
T=virtual_sa_userdelivery defer (-24): Transport filter process
failed (127): unable to execute command
2005-09-08 14:01:06 1EDQJO-0006tU-Un <MattPike@???>:
virtual_sa_userdelivery transport output: An error was detected while
processing a file of BSMTP input.
2005-09-08 14:01:06 1EDQJO-0006tU-Un == mattpike@???
<MattPike@???> R=virtual_sa_user
T=virtual_sa_userdelivery defer (-24): Transport filter process
failed (127): unable to execute command