Mailgun is an email delivery platform for developers, part of Sinch. It provides APIs and SMTP relay to send, receive, track, and optimize email at scale.

Send - email API and SMTP relay to send, receive, and track emails.
Optimize - deliverability tools and inbox placement to keep mail out of spam.
Validate - email address verification by API or bulk list upload.
Inspect - email preview and testing with accessibility checks.
Enterprise options - dedicated support, consulting, and high-volume sending.

It is used by developers and businesses sending transactional and marketing email, including high-volume senders that need deliverability tooling.

Mailgun offers a free signup with up to 100 emails per day free.