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Direct SMTP Connection vs. Custom SMTP Relay


Greenhouse uses Mailgun, a third-party service, to send customer emails. However, Greenhouse provides two SMTP setup options for customers who prefer to send emails via their own organization's architecture. In this article, we'll provide a brief overview of the two SMTP setup options: Direct SMTP Connect and Custom SMTP Relay.


Note: By using a Direct SMTP Connection or a Custom SMTP Relay to send emails, your internal IT team will be required to troubleshoot any email related issues. Our support team will not be able to hep diagnose errors or issues that occur after the email has been triggered from Greenhouse.

For more information regarding either SMTP setup, please contact your Customer Success Manager.


Direct SMTP Connection

When using a Direct SMTP Connection, you will need to provide Greenhouse with connection information (host/port of their email server) and a set of credentials for the Mail Transfer Agent managing mail for your domain (username/password). We use this information to connect remotely to your server via SMTP Authentication and then to send emails on your behalf.  


Since Direct SMTP Connection requires Greenhouse to log into your server directly, Greenhouse requires the following information to set up this SMTP option:

  • Host name of the server
  • Port Number
  • Username of the server (not the credentials of an individual user's mailbox)
  • Password of the server (not the credentials of an individual user's mailbox)


Custom SMTP Relay 

When using a Custom SMTP Relay, Greenhouse does not authenticate into your server directly. Greenhouse will send all emails to one of our SMTP servers first which are then relayed to your organization's mail server. The relay allows us to guarantee that all of our SMTP messages originate from one of a set of known IP addresses. You will need to whitelist each of the IP addresses and accept all messages sent by these machines.


For a Custom SMTP Relay, Greenhouse requires you to provide:

  • Host name of the server
  • Port Number

Additionally, your organization will need to whitelist a set of IP addresses that we provide. 

There is an additional charge for a Custom SMTP Relay, since it requires custom development and dedicated testing from our technical support team. For more information about the list of IP addresses that will need to be whitelisted for full functionality, please click here.