You can connect to the server the final recipient gets her mail from directly. The administrator of this server can decide to use another server as a relay for the domain you want to send mail to. This server in turn can be configured to use another relay, and so on. Spammers and relays The SMTP protocol allows for this relaying since it was deemed useful. Unfortunately, this feature can be abused.
Spammers can relay through another server to cover their identity.
As we have seen when we looked at the headers of unsolicited email, every mail server needs to insert the Received line with a time-stamp into the header. In this line also appears from where the server accepted the message, that is the domain name the client stated as an argument to the HELO command. This is essential to trace back the source of spam, as we have seen. If the server does not include this additional information into the header this is rather unfortunate. We then have hardly a way to find the spammer's ISP other than looking for contact information in the body of the message and doing further investigation.