Wow, thanks for the super quick reply

the log now says:
@4000000045f13e1e1633384c new msg 6031156
@4000000045f13e1e16334404 info msg 6031156: bytes 341 from <anonymous@sitescom.eu> qp 4307 uid 102
@4000000045f13e1e167b2e2c starting delivery 385: msg 6031156 to remote ks@sitescom.eu
@4000000045f13e1e167b6cac status: local 0/10 remote 1/20
@4000000045f13e1e29c64c3c delivery 385: failure: 80.168.44.12_does_not_like_recipient./Remote_host_said:_550-Verification_failed_for_<anonymous@sitescom.eu>/550-"anonymous@sitescom.eu"_is_not_a_known_user/550_Sender_verify_failed/Giving_up_on_80.168.44.12./
@4000000045f13e1e29c95d64 status: local 0/10 remote 0/20
@4000000045f13e1e2a284a2c bounce msg 6031156 qp 4309
@4000000045f13e1e2a2851fc end msg 6031156
@4000000045f13e1e2a2ca75c new msg 6031305
@4000000045f13e1e2a2caf2c info msg 6031305: bytes 1033 from <> qp 4309 uid 406
@4000000045f13e1e2a74668c starting delivery 386: msg 6031305 to remote anonymous@sitescom.eu
@4000000045f13e1e2a747244 status: local 0/10 remote 1/20
@4000000045f13e1e36d15534 delivery 386: failure: 80.168.44.12_does_not_like_recipient./Remote_host_said:_550_"anonymous@sitescom.eu"_is_not_a_known_user/Giving_up_on_80.168.44.12./
@4000000045f13e1e36d49d0c status: local 0/10 remote 0/20
@4000000045f13e1e3728eefc bounce msg 6031305 qp 4311
@4000000045f13e1e372a7d6c end msg 6031305
@4000000045f13e1e372ea004 new msg 6031300
@4000000045f13e1e372f9a04 info msg 6031300: bytes 1565 from <#@[]> qp 4311 uid 406
@4000000045f13e1e3765c14c starting delivery 387: msg 6031300 to remote postmaster@sitescom.eu
@4000000045f13e1e37671cf4 status: local 0/10 remote 1/20
@4000000045f13e1f138ef734 delivery 387: success: 80.168.44.12_accepted_message./Remote_host_said:_250_OK_id=1HPcov-00056O-0V/
@4000000045f13e1f138f02ec status: local 0/10 remote 0/20
@4000000045f13e1f138f0abc end msg 6031300
So at least the mail was sent as far as the remote mail server.
Now I just need to figure out why the mail is sent from "anonymous@sitescom.eu" and not from the address I put in the from header in the php script :
mail('ks@sitescom.eu', 'subject', 'message', "From: siteadmin@sitescom.eu\n");
Once again, thankyou for your amazingly quick response.