Mailgun

Mailgun was built as an API for developers who wanted to send and receive email as part of an application rather than manage a mail server themselves. Beyond the standard send API and SMTP relay, it supports routing rules that can forward, store, or forward-and-store inbound mail based on conditions you set, which matters for anything that needs to react to a reply, a support ticket system or a parsing pipeline for example. Email validation is built into the API too, catching typo'd or dead addresses before you send to them.

Mailgun bills by email volume rather than contacts. A free plan covers 100 emails a day, Basic runs $15 a month for 10,000 emails, and Foundation and Scale tiers step up to 50,000 and 100,000 emails a month respectively, with per-thousand overage rates that drop as you move up tiers. One data point worth knowing: independent price tracking has found no detected increases in Mailgun's published rates since 2018, unusually stable for this market. It suits engineering teams who want an API-first tool with inbound processing and don't need a marketing-style campaign builder.