What is an inbound email API?
Sending email is the easy half. An inbound email API is the other half: it receives mail for your domain and hands it to your code. Here is how it works and what to check before you pick one.
By Arin Prajapati, founder, 3 min read
The short definition
An inbound email API receives email addressed to your domain and delivers it to your application as structured data. Instead of running your own mail server, you point a DNS record at the provider and read the mail through HTTP: either each message is posted to your webhook, or it is stored in an inbox you list and fetch.
How it works
- DNS. You add an
MXrecord so that other mail servers send your domain's mail to the provider. - SMTP. The provider's server accepts the message, checks SPF, DKIM and DMARC, and filters spam.
- Parsing. It splits the raw message into sender, subject, text, HTML, attachments and thread.
- Delivery. You get it by webhook, by API, or both.
If those terms are new, SPF, DKIM and DMARC explained shows the records and what each result means.
Webhook or stored inbox
There are two styles. A webhook posts each message to a URL you run. It is simple, but you must handle retries, ordering and downtime, and the mail exists only in that one request. A stored inbox keeps the message and lets you list, search, thread and reply later, so a missed webhook loses nothing. Many products offer both. Runearc stores inbound mail in inboxes you read over REST, IMAP or MCP, and also sends signed webhooks.
What to check
- Whether received mail counts against your sending quota, and how long mail is kept.
- Whether you can use your own domain on the plan you will pay for.
- Whether messages arrive with authentication results and extracted codes and links.
- How webhooks are signed and retried.
- Whether the same account can send, so replies stay in one thread.
Runearc is currently free. The paid Runearc prices below are planned and come from our pricing page; billing is not live yet.
For a wider comparison by job, see the best email API for developers. To receive support mail on your own domain without a per-seat mailbox, read support@ on your own domain.
Questions
What is an inbound email API?
An inbound email API receives email for your domain and gives it to your code as data, either by posting each message to a webhook or by storing it in an inbox you read over HTTP. It saves you from running a mail server.
How do I receive email in my app?
Point your domain's MX record at an inbound provider, then either accept its webhook or read messages from its API. Verify SPF, DKIM and DMARC results on each message before you trust a link in it.
Is receiving email harder than sending it?
Different, not harder. You need an MX record, a server that accepts SMTP, spam handling, and parsing of MIME, attachments and threads. An inbound API does those parts for you.
Does received mail count against my sending limit?
It depends on the provider. Some count each received email against the same quota as sent mail; others, including Runearc, count sent recipients only. Check this before you choose.
Arin Prajapati, founder of RunearcA software engineer in Mumbai who builds Runearc's mail servers, API and site. More about Arin and why he built Runearc.
Read next
- The best email API for developers depends on the jobMost roundups rank email APIs as if they did one thing. They do five. Here is how to choose by the job you have, who fits each one, and where Runearc is the wrong pick.
- How email authentication works: SPF, DKIM and DMARC explained with real recordsThree DNS records decide whether your mail is trusted. Each one answers a different question. Here they are, one at a time, using the exact records Runearc generates for a new domain.
- support@ on your own domain, without Google WorkspaceMost small SaaS products need two or three shared addresses, not a seat per person. Here is how to run support@ on your domain, keep the mail you already have, and answer from a shared inbox or Apple Mail.
Put Runearc to work
Get inboxes, sending, your own domains and MCP in one workspace. The free plan includes 3 inboxes, 3,000 emails a month and 3 custom domains, with no card.