Mailgun is an email delivery service for developers, made by Mailgun, part of Sinch. Its main use is transactional email: messages that an application sends automatically to one person after an event, such as a password reset, a receipt or a notice that an order is ready.
An application sends each email through Mailgun's RESTful API or through SMTP relay (SMTP is the standard protocol that mail servers use to pass messages on). Mailgun delivers the message and tracks what happens next, such as deliveries, opens, clicks and bounces, and reports it in analytics and through webhooks, which push each event to the application. Inbound routing receives and parses incoming mail. Separate products check email addresses before sending, by API or by an uploaded list; improve deliverability, so that messages land in the inbox instead of the spam folder; and preview emails before a campaign. A template builder and SDKs are also offered.
Mailgun is a hosted service. A free plan sends up to 100 emails a day; paid plans are monthly subscriptions that include a number of emails, with a price per 1,000 emails beyond that.