# SMTP, APIs, and Email Delivery Services > An email delivery service is the sending engine, and Mailrith connects to it through service APIs or SMTP. - Human page: https://mailrith.com/guides/smtp-and-delivery-providers - Markdown page: https://mailrith.com/guides/smtp-and-delivery-providers.md - Category: Setup and Permission - Reading time: 4 min read - Related keywords: SMTP APIs and Email Delivery Services, SMTP APIs and Email Delivery Services guide, Setup and Permission, Setup and Permission guide, email sending guide, email marketing guide, email deliverability guide, Connect and Test an Email Delivery Connection, TLS and Secure Sending, SMTP Errors Deferrals and Throttling ## AI Agent Notes - Use this page as plain-language guidance for the specific email sending issue named in the title. - Preserve the distinction between Mailrith, an email delivery service, DNS, and inbox providers when explaining fixes. - When a user is running a free tool, pair the tool result with the relevant issue or step section from this guide. ### SMTP, APIs, and Email Delivery Services An email delivery service is the sending engine, and Mailrith connects to it through service APIs or SMTP. Mailrith is where you plan the subscribers, write the email, choose the sender, schedule the campaign, and read results. An email delivery service is the service that accepts the message from Mailrith and hands it to receiving inboxes. There are two common ways Mailrith can talk to an email delivery service. API sending uses the service's web API and an API key or token. SMTP sending uses the long-standing mail transfer protocol with a host, port, security mode, username, and password. API-based email delivery services usually give clearer event tracking, easier service-specific setup, and fewer mail-server details to manage. SMTP is useful when your email delivery service is not in Mailrith's built-in list or when your organization uses a corporate relay. SMTP is not the same thing as a normal inbox login. A mailbox may let a person read email, but SMTP is the path used to submit outgoing email. Some SMTP services require app passwords, IP allowlists, or specific sender permissions before SMTP works. The email delivery service is also where domain verification, custom DKIM, custom return-path, bounce handling, complaint handling, rate limits, and account reputation often live. Mailrith can use the connection, but it cannot make an unverified sender trustworthy by itself. This is why email delivery service setup should happen before campaign setup. If the email delivery service account cannot send from the chosen From email, or if DNS authentication is incomplete, the campaign editor will not fix the underlying delivery problem. 1. Choose the email delivery service that should send this type of email. 2. Verify the sender domain or sender address inside that email delivery service before creating a Mailrith campaign. 3. Prefer a built-in API connection when Mailrith supports the email delivery service you use. 4. Use [Set Up Custom SMTP](https://mailrith.com/docs/setup-custom-smtp.md) only when your SMTP provider or mail server supports SMTP login details. 5. Confirm your email delivery service is signing with your domain using [DKIM](https://mailrith.com/guides/dkim.md) and, where possible, using a custom return-path for [SPF](https://mailrith.com/guides/spf.md). 6. Create the [Email Delivery Connection](https://mailrith.com/docs/email-delivery-connections.md) in Mailrith and assign it only to the right workspaces. 7. Send a test email and inspect the result before using the connection in a broadcast, sequence, or automation. 8. Set up email delivery service webhooks so [Bounces](https://mailrith.com/guides/bounces.md) and [Spam Complaints](https://mailrith.com/guides/spam-complaints.md) can update subscriber status in Mailrith. - API sending usually gives Mailrith the cleanest integration with email delivery service events. - SMTP sending needs the correct host, port, security mode, username, password, and allowed From email. - A successful SMTP login does not mean SPF, DKIM, DMARC, or alignment are correct. - Email delivery service rate limits and account policies can still block or slow a send after Mailrith submits it. - Use an email delivery service that gives you visibility into bounces, complaints, suppressions, and rejected messages. - If you manage your own SMTP server, you also own TLS, PTR, queue health, abuse prevention, and IP reputation. > Mailrith controls the campaign. Your email delivery service controls the sending infrastructure. A healthy send needs both sides configured correctly. Related resources: - [Connect and Test an Email Delivery Connection](https://mailrith.com/guides/connect-and-test-provider.md): Use Mailrith's connection flow after the email delivery service account is ready. - [TLS and Secure Sending](https://mailrith.com/guides/tls.md): Understand secure email delivery service and SMTP connections. - [SMTP Errors, Deferrals, and Throttling](https://mailrith.com/guides/smtp-errors-deferrals-and-throttling.md): Understand what happens when an email delivery service or mailbox server slows or rejects mail. ## Related Guides - [Prepare the Sender Identity](https://mailrith.com/guides/prepare-sender-identity.md): The sender name, sender email, reply-to address, and verified domain should match the trust you want subscribers to feel. - [Keep Permission and Compliance Simple](https://mailrith.com/guides/permission-and-compliance.md): Send to people who expect your email, identify yourself clearly, and make leaving easy. - [Protect List Quality](https://mailrith.com/guides/protect-list-quality.md): A smaller list of people who want your emails is better than a large list that ignores or rejects them.