[exim] vanishing emails

Im sending emails from a PHP app -> exim (instead of sendmail). the logs seem
to show that the mesage has been sent , but they arent arriving the other
end.
Anyone know how to start troubleshooting this (if i can) ?

grenada # tail /var/log/exim/exim_main.log
2005-07-15 00:52:04 End queue run: pid=26130
2005-07-15 01:03:41 1DtDg5-0006on-2t <= apache@??? U=apache
P=local S=2524
2005-07-15 01:03:41 1DtDg5-0006on-2t => xxxxx@??? R=send_to_gateway
T=remote_smtp H=smtp.isp.co.uk [xx.xx.x.x]
2005-07-15 01:03:41 1DtDg5-0006on-2t Completed
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p cooper