Spammers can send email from a spoofed account using your own domain name and send it to you. For example, a spammer can send an email from anywhere on the internet from bob@example.com to joe@example.com . Since mail servers see this as an inbound delivery, they do not require SMTP Authentication to deliver in this way.

The best way to protect yourself from these types of spam is to do the following:

1. Require SMTP authentication for your domain (Top Network requires SMTP autrhentication by default);
2. Add an SPF record for your web site in DNS (see http://help.smartertools.com/smartermail/v5/Topics/SystemAdmin/Misc/DNSSettings.aspx);
3. Enable / Configure / Require DomainKeys (see http://help.smartertools.com/smartermail/v5/Topics/DomainAdmin/Settings/DomainKeys.aspx).