a company that sends emails on behalf of others. Following are the most notable types of sources:

• ESPs – offer the ability to send bulk (marketing and/or transactional) emails with their own domain
• ISPs – offer the ability to receive and send emails with your own domain (business or personal communication, but not bulk messaging)
• Other services allow you to send emails with your own domain. Those can be support/ticketing systems, payment providers, emerchant services, etc.