Implied opt-in is a strategy is based on the assumption that since people have given you their email addresses, you have permission to send them at least one email. And if you say in it that you would like to follow up that email with further marketing communications of a specific kind unless they objected, then you have obtained their permission to continue.
Some consider this to be a gray area of permission marketing, and others will consider it to be a useful tool to be used in certain situations.
If used carefully, B2B marketers can find it an effective way to legitimately increase their email files. However, check first if your country's legislation allows this practice.
Read an interesting article about implied opt-in written by Page Duffy, principal consultant with JPD Associates.