An email delivery service is a third-party service that handles the sending of transactional and/or marketing emails. Transactional emails are things like purchase receipts or contact forms on your website that send to a single user. Marketing emails are bulk emails that are sent for marketing purposes. The focus of this article will be transactional emails.
Let's start by looking at what's going on when a user does something that triggers an email on your website. That email can get sent by the server itself or by a third-party service. It may seem simple enough to let the server send it and not worry about it. However, that leaves you the website owner in the dark as to whether the email was sent successfully or failed to send. If the emails sent by the server fail it's usually a "silent" failure. The email just doesn't get sent and no one is the wiser (unless you've configured things differently and are monitoring server logs). If the unsent email happens to be a contact form that should reach you, that "silent" failure is a problem. There is also the issue of your server emails possibly getting flagged as SPAM and winding up in a potential customers SPAM folder. Small things like this don't instill confidence in your business.
With a transactional email setup you'd configure your website to send email via an outside service. Most content management systems provide either a native way to implement this via SMTP (Simple Mail Transfer Protocol) email or a plugin to implement it. In Expression Engine and Craft SMTP is built in so it's super-easy as long as your host supports it. Under this setup the email is relayed via SMTP to the transaction email service which accepts it and then handles sending that to the correct recipient(s).
A transactional email service provides the following benefits for your users and you as the website owner.
If your website is sending emails - a contact form, a user notification - then I think you should have a transactional email service in place. The cost is low and is far outweighed by the benefit. There are a number of different services out there but if you'd like a recommendation then look at Postmark. They provide a solid service at a great price. Contact us if you have other questions or want help getting a transactional service in place.