“In recent years ISP reputation has been assigned to IP addresses and domains. As ISPs continue to finesse algorithms they will recognize that one person’s spam is another person’s ham. Filter rules will work increasingly at the personal level and may lead to filtering of permission-based email from the inboxes of the users that are consistently ignoring the brand.”
Google is constantly changing its algorithms to supply users with the best search results; this also applies to the filter algorithms defending your customer’s inbox. You should definitely not spam your audience, however what is/is not spam is in constant flux and different for different users. Stay familiar with ISPs best use practices for sending email marketing.