hmmmnnn, interesting. port 587? i googled smtp port 587 and found rfc2476 which says some things about how these types of mailservers (listening on port 587) can be configured:
http://www.ietf.org/rfc/rfc2476.txt
3.2. Message Rejection and Bouncing
MTAs and MSAs MAY implement message rejection rules that rely in part
on whether the message is a submission or a relay.
For example, some sites might configure their MTA to reject all RCPT
TOs for messages that do not reference local users, and configure
their MSA to reject all message submissions that do not come from
authorized users, based on IP address, or authenticated identity.
NOTE: It is better to reject a message than to risk sending one that
is damaged. This is especially true for problems that are
correctable by the MUA, for example, an invalid ‘From’ field.
If an MSA is not able to determine a return path to the submitting
user, from a valid MAIL FROM, a valid source IP address, or based on
authenticated identity, then the MSA SHOULD immediately reject the
message. A message can be immediately rejected by returning a 550
code to the MAIL FROM command.
does that sound familiar?
so, who is running your mailserver? can you get the log to see what it says there?