How to use MailGun as a Smarthost

Introduction

Many providers now block outbound connections on port 25, meaning a cPanel system would need to configure a smart host to allow outbound emails to function. This resource covers how to set up a smarthost using a common 3rd-party mail provider, MailGun.

MailGun has numerous plans, including a pay-as-you-go plan. Their free tier offers 10,000 emails, 100 email validations, and 5-day log retention.

Procedure

The MailGun smarthost setup instructions are similar to the SendGrid smarthost setup instructions. The main difference with MailGun is that it uses "ROUTERSTART" instead of "PREROUTERS".

To configure MailGun on your server: 

1. Sign up for MailGun, if you still need to do so.
2. Log in to WHM, and navigate to "WHM / Service Configuration / Exim Configuration Manager / Advanced Editor." On this page, search for "Section: AUTH". In the text box, input the following:
   

   mailgun_login:
    driver = plaintext
    public_name = LOGIN
    hide client_send = : Default-SMTP-Login:Default-Password

   NOTE: The Default-SMTP-Login and Default-Password are found in your MailGun account in the Domain Settings section.
3. Next, go to the "Section: ROUTERSTART" and input the following:
   • On v108 or above
     

     mailgun:
      driver = manualroute
      domains = ! +local_domains  : ! +manualmx_domains
     .ifdef SRSENABLED
      # if outbound, and forwarding has been done, use an alternate transport
      transport = ${if eq {$local_part@$domain} \
               {$original_local_part@$original_domain} \
               {mailgun_transport} {mailgun_forwarded_transport}}
     .else
      transport = mailgun_transport
     .endif
     route_list = "* smtp.mailgun.org::587 byname"
     host_find_failed = defer
     no_more

   • On v106 or below
     

     mailgun:
      driver = manualroute
      domains = ! +local_domains  : ! +manualmx_domains
      transport = mailgun_transport
      route_list = "* smtp.mailgun.org::587 byname"
      host_find_failed = defer
      no_more

4. Then go to "Section: TRANSPORTSTART." Input the following into the text box:
   

    mailgun_transport:
      driver = smtp
      hosts = smtp.mailgun.org
      hosts_require_auth = smtp.mailgun.org
      hosts_require_tls = smtp.mailgun.org

5. On v108 or above, additionally, add the following to the "TRANSPORTSTART" section:
   

   .ifdef SRSENABLED
   mailgun_forwarded_transport:
    driver = smtp
    hosts = smtp.mailgun.org
    hosts_require_auth = smtp.mailgun.org
    hosts_require_tls = smtp.mailgun.org
    max_rcpt = 1
    return_path = ${srs_encode {SRS_SECRET} {$return_path} {$original_domain}}
   .endif

6. Go to the bottom of the page and click "Save". The changes will get added to the Exim configuration and will automatically restart the Exim service.
7. Try sending a test email using one of the Webmail applications. If the email does not leave the server, you must examine the /var/log/exim_mainlog file to determine the cause of the issue.

Notes:

• This guide is a task intended for system administrators. cPanel's Technical Analysts cannot provide assistance in configuring a server's smarthost setup.
• Configuring a smart host will cause all outbound emails to go through the configured provider. The scope of this document does not cover how to configure a subset of domains to go through a smart host; the server's system administrator must configure this.