Usually when you see the words unlisted recipients within your email program it's probably unrequested spam. I subscribe to many newsletters and this means they have my email address and/or my name. When I receive my newsletter, it is personalized with my name and/or email address. Most of the newsletters I receive but did not request arrive in my mailbox with the words, "Unlisted Recipients" within my email programs' TO field. I simply filter those newsletters to my trash bin or immediately delete them off my server. There is a possibility that some ISP's filter email with the words "Unlisted Recipient" before they reach my mailbox. As an email marketer you had better be sensitive to what the Internet Service Providers consider SPAM along with what they look for. I don't know all of the details, but there some ISP's that won't let email pass through with "unlisted recipients" showing in the TO of your email program field.
Here comes the quick and dirty Personalization tactic. If you know your subscribers by name you can have your email program send out messages with their name and email address in the To field.
Why trip the SPAM filters when you're a legitimate e-marketer? Rhetorical question. Know the actions that would ensure your email messages get to their final destination.
Personalized your mailout by using a subscriber's email address or name. I'm not saying that using "Unlisted Recipients" will cause your site to be rejected by some subscribers but why test the waters?
Happy Marketing!
Chris McClean