You can warm up a Mailgun sender through MailSlurp while keeping Mailgun as the sending service. The connection uses your verified domain and a domain sending key, with paced activity and delivery observations to help you review the sender as it grows. The important setup detail is that the domain, key and API region belong together. Once that connection works, you'll have a much clearer view of whether a problem occurs before Mailgun accepts the message or farther along its journey. ## Which part of Mailgun are you warming up? A new sending domain and a new dedicated IP address are separate changes. Mailgun's automatic IP warmup controls traffic through a dedicated IP and can route additional traffic through other available IPs. Its [automatic IP warmup documentation](https://help.mailgun.com/hc/en-us/articles/8305731694747-Automatic-IP-Warmup) describes that behavior. MailSlurp's warmup program uses the sending connection you configure and lets you review its activity and delivery observations. When dedicated-IP warmup is also enabled in Mailgun, the underlying IP routing remains relevant: a successful send alone doesn't prove which IP carried it. Provider logs and received headers help establish that. If you're deciding whether a change affects the mailbox, domain or IP, the [IP warmup versus domain warmup guide](/blog/ip-warmup-vs-domain-warmup/) explains the terminology. Here, we'll focus on connecting a real Mailgun sender and checking its first scheduled messages. ## Match the domain, region and sending key MailSlurp's named **Mailgun** connection uses the messages HTTP API. Its credential is a Mailgun domain sending key; an SMTP password belongs to a different connection method. - **Sending domain:** the exact verified domain in Mailgun, including any subdomain. - **Domain sending key:** a key created for that domain with permission to send messages. - **API region:** US or EU, matching where the domain is configured. - **From address:** the sender identity intended for this program. Mailgun uses `api.mailgun.net` for US domains and `api.eu.mailgun.net` for EU domains. The region follows the domain's configuration, even if the person setting up MailSlurp happens to be sitting on another continent. Mailgun's [API overview](https://documentation.mailgun.com/docs/mailgun/api-reference/api-overview) documents the regional endpoints. A domain sending key is narrower than the primary account key: it permits message submission for the domain where it was created. Mailgun's [authentication guide](https://documentation.mailgun.com/docs/mailgun/api-reference/mg-auth) describes how to create one under the domain's Sending API keys settings. Give it a recognizable name so the team can identify the connection later, and keep the secret out of shared troubleshooting notes. A useful preflight note is simply the domain, its region, the key's descriptive name and the intended From address. Four values written down once can save a surprising amount of credential re-entry. ## Connect Mailgun in MailSlurp 1. Open **Email warmup** and choose **Create warmup**. 2. Select **Mailgun**, or reuse an existing Mailgun sending connection. 3. Enter the domain sending key, domain, region and sender details requested by the form. 4. Verify the credential and complete the test delivery. 5. Choose the new-sender or sender-repair goal, then configure the schedule and coverage. 6. Review the program and start it when the test delivery is working. The [Mailgun connection reference](/docs/email-warmup/mailgun/) provides the short field checklist. If the domain is still being configured, our [email subdomain guide](/guides/email-subdomains/) explains the visible From address, return path and DKIM signing domain before you begin. A verified credential establishes that the connection can authenticate. A received test lets you inspect the actual message. For custom-domain mail, those are useful separate checks: the application may authenticate successfully while using an unexpected sender identity. {{LANDING_SHORTCODE:POST_CTA_BANNER:%7B%22title%22%3A%22Warm%20up%20your%20Mailgun%20sender%20with%20MailSlurp%22%2C%22content%22%3A%22Connect%20the%20sending%20domain%20you%20already%20use%2C%20verify%20delivery%2C%20and%20follow%20a%20paced%20program%20with%20sender%20activity%20and%20available%20provider%20placement%20checks.%22%2C%22buttonHref%22%3A%22%2Fproduct%2Femail-warmup%2F%22%2C%22buttonText%22%3A%22Explore%20email%20warmup%22%2C%22buttonTheme%22%3A%22slate%22%2C%22image%22%3A%22asset%3Aproduct%2Femail-warmup%2Fsender-provider-cluster-v2.png%22%2C%22imageAlt%22%3A%22Supported%20email%20providers%20in%20MailSlurp's%20sender%20connection%20artwork%22%2C%22imageWidth%22%3A%221536%22%2C%22imageHeight%22%3A%221024%22%2C%22imagePosition%22%3A%22%22%7D}} ## Why accepted and delivered tell different stories Mailgun's `accepted` event means it has queued the message for sending. Its `delivered` event means the receiving mail server accepted it. Neither event identifies the final inbox folder. These definitions come from [Mailgun's event reference](https://documentation.mailgun.com/docs/mailgun/user-manual/events/events). A useful investigation follows one message through those stages. The same timestamp, recipient and provider message ID help correlate events, while received headers confirm the sender identity. MailSlurp's controlled-inbox receipt shows that a message arrived there; completed provider placement checks show the observed inbox or spam outcome for their sampled destinations. For example, a Mailgun delivered event alongside a completed spam-placement result isn't a contradiction. The receiving server accepted the email and subsequently classified it as spam. Replacing the API key would address neither part of that result. The broader [Mailgun email testing guide](/mailgun-email-testing/) covers application assertions, routes and webhooks. For a warmup review, the narrower question is whether the configured sender is working and how its completed delivery observations change over time. ## Troubleshooting a stalled Mailgun warmup The provider response is more useful than a generic "failed" label. A short diagnosis can begin with the stage that stopped: - **Credential verification fails:** confirm the key type and whether it remains active. - **Domain or send request rejected:** compare the exact domain, region and key scope with the connection. - **Send accepted, result pending:** check later provider events and the observation's completion state. - **Temporary delivery failure:** read the receiving server's response and review provider retry activity. - **Repeated completed spam results:** inspect authentication, sender history and representative message content. Those are investigation paths, not a promise that every failure has the same cause. Copying the response text, time and last successful test into a note makes it easier to distinguish a configuration change from a destination-specific problem. After a domain or key correction, another verified test delivery gives you a clear point to resume from. A pause is also useful while investigating repeated failures; it keeps the next review from being buried under more of the same error. ## Keep the program connected to real sending Other services using the Mailgun domain still contribute email volume. A newsletter platform, an application and the warmup program may share the same account even though each has its own schedule. MailSlurp lets you customize the daily target, ramp duration, maintenance and sending window. The [warmup schedule explanation](/blog/email-warmup/#which-email-warmup-schedule-makes-sense) helps choose a pace that fits the intended workload. Provider limits and any dedicated-IP routing should be reviewed alongside that plan. Warmup content and a finished campaign serve different checks. MailSlurp supports managed warmup content and custom plain-text content with constraints on links, tracking and media. A separate placement test using the actual newsletter or application email is useful before a larger send, particularly when it contains a new template or tracking domain. The [warmup results guide](/guides/email-warmup-inbox-placement/) walks through sample size, partial checks and baseline comparisons. When the connection details are ready, [start your Mailgun warmup in MailSlurp](https://app.mailslurp.com/sign-up/?onboardingRequest=deliverability).