Not having permission
Unfortunately there are still businesses that send emails to people who have not given permission to do so. It is not wise to assume people want to receive your emails. One of the consequences could be that people will report your email as spam. This could even lead to spam filtering or blacklisting.
Blacklisting is invented by Internet Service Providers (ISPs) in order to reduce spam. Your emails will be blocked by an ISP as soon as several people report you. Wikipedia describes their spam blacklist as “a control mechanism that prevents an external link from being added to a Wikipedia page when the URL matches one listed at MediaWiki:Spam-blacklist. It mostly lists spammers, but also includes URL redirection services (which could otherwise be used to bypass blacklisting).”