Which of the following is most likely to cause an email to be blocked by an ISP or corporate filter: Content, volume, bounce percentage or black list-based filters?

Answer: In general content/HTML code and black lists are the most likely to trap your emails. Generally only organizations with very large lists – sending several hundred thousands of emails to a single ISP – are likely to have their emails filtered based on volume. For our large volume clients, EmailLabs limits the volume of emails sent per hour to certain ISPs. If you conduct ongoing list hygiene, including removing hard bounces, unsubscribes, people filing spam complaints and minimize email addresses with bad syntax upon opting in, your bounce percentage should fall well below the danger zone with ISPs.