Spammers commonly put their address list in the BCC: field, which will not show up in the headers. Many servers and mail clients will bounce messages with empty To: fields. The spammer may put a dummy address into the To: field, or the To: field may be filled by the first name on a particular SPAM run. If a spammer has a huge list of addresses for a SPAM run, that list may be parsed into groups of less than 100 in order to get it by the server SPAM filters. The first e-mail address on the list will go in the To: field, the rest of the addresses will be put in the Bcc: field. If your e-mail address is further down the list, it will end up in the Bcc: section. The e-mail will then appear to have been sent to someone else.