hmm ... those headers aren't giving us anything useful... can you explain how your email gets sent... do you connect each user directly using pop3 accounts?
it might be worth contacting your ISP/host if you get your email as part of a hosting package:
VIANetworks
620 Birchwood Boulevard
Warrington
Cheshire
WA3 7QZ
GREAT BRITAIN (UK)
Admin Email:
domains@VIANETWORKS.CO.UK
Admin Phone: +44192548444
Admin Fax: +441925484899