If you have your self-hosted wordpress blog or online shop, you probably send emails to your potential customers. Every time you send an email from your blog, it will be sent through your own server’s IP. Its delivery will be partially decided on your domain reputation and the sender’s IP reputation. If you use shared hosting, than not just your sending influences your IP reputation, but all the other companies who use the same server. Your sender score will be determined by your worst neighbour’s sender score. If there is one spammer from 100 tenants, you will be considered as spammer as well.
If you have a hosted e-commerce solution, than email sending will be built into the price you pay. I assure you, it its always cheaper and more reliable to send with third party endpoints, than pay for email traffic through your webshop hosting company.
Third party endpoints can reliably delivery your emails. They use IP pools to do it, which means, that you are sending from 2-20 IP addresses at the same time. They rigorously manage their own IP reputation, which also means, that bad concent, high bounce rate and spam is not welcome. If your sending practices are weak, (you have many bounces, few opens, spam looking offers) than they will kick you out.