why your e-mails may not arrive in your customers’ inboxes. They may have been inadvertently/incorrectly categorized as spam by the ISP (Internet Service Provider), or filtered into a junk-mail folder by a web-based or desktop e-mail client.

the old definition of spam still lingers. For example, someone might be chided for “spamming a mailing list” when they are not a robot attempting to evade a filter, nor a commercial mailer advertising a product or a service.