Email service provider

What it means

M3AAWG defines an ESP as a company that offers services to send email at volume on behalf of its customers. RFC 6449 makes the same distinction and notes that M3AAWG uses ESP only in that sense, never for a mailbox provider that hosts end users' inboxes.

An ESP's reputation is partly shared by its customers: on shared IPs and shared domains, one customer's practices affect the others, which is why M3AAWG requires ESPs to vet customers before and after they send, handle bounces and feedback loops, and encourage each customer to authenticate with its own domain.

Google's sender guidelines say that if you have delivery issues with mail sent by a provider, you should check that the provider meets Gmail's requirements.