Email Service Provider
When you send an email it goes through an outgoing email server. That email server is operated by your email service provider.
Generally speaking, email service providers fall into one or more of the following categories:
- Hosted email
- Newsletters/bulk email
- Transactional email
This category is the one most people consider when talking about email. When you write an email in your email program and hit "Send", that email is processed by someone. This could be your Internet Service Provider acting as your ESP as well, or it could be a dedicated provider like Gmail, Outlook.com or Fastmail.
When sending out bulk emails, ie where the same email need to reach a bunch of recipients at once, your hosted email provider doesn't cut it.
In this case you want to use a dedicated bulk email provider that specializes in delivering massive amounts of emails as quickly as possible. In this category, we find providers like MailChimp or ActiveCampaign. It is worth noting that many of these also double as transactional email providers.
Sending an email on behalf of your web application or website because a user took some action is called transactional email. Email Service Providers like Mailgun, SendGrid, and Postmark have carved out niches here.