Why can't I add someones name to my mailing list if they give me their card at a networking event?

There seems to be a great deal of confusion about consent for sending marketing emails.

Under the Data Protection Act 1998 opt-out (or similar) consent is generally sufficient in the case of marketing emails involving non-sensitive personal data. However, express or opt-in consent would be required for any direct marketing communications which involve the processing of sensitive personal data, such as data relating to ethnicity, politics or medical conditions.

Opt-in or equivalent consent is required under the Privacy Regulations for marketing emails sent to individual subscribers, unless the soft opt-in provisions apply i.e..  (NB the Privacy Regulations do not use the terms “opt-in” and “opt-out”.)

You should also check the requirements of your email service provider’s terms and conditions. These often required a more stringent standard of consent than the general law.

So you will need to do something more than simply harvesting email addresses to legitimately send marketing emails.