Mailgun is a third-party email provider Greenhouse Recruiting uses to send emails. All emails from Greenhouse are sent via Mailgun. This includes (but is not limited to) emails such as new user invitations, notifications, emails to candidates, and @mention messages between users.
As a part of the Greenhouse Recruiting implementation process, we ask all organizations to register their domain within the Greenhouse application and complete the email verification process. This process registers your domain with Mailgun, who in turn generates DNS records (SPF, DKIM, and sometimes MX records) to be added to your domain host.
The primary purpose of adding these records to your domain host is to expressly permit Greenhouse Recruiting (via Mailgun) to send emails on behalf of your domain, which helps prevent emails from going to spam.
All outbound emails from Greenhouse Recruiting are sent through Mailgun whether or not a domain is verified. Any emails sent through Greenhouse using our email provider from an unverified domain will be sent through outbound-mail.greenhouse.io instead of through your domain.
A flag will appear in the from: address of your emails that reads via greenhouse.io. Since the domain that emails are sent from do not match the domain in the from: address, these emails will likely go to spam.
The SPF, DKIM, and MX records are generated by Mailgun. However, the SPF record only permits emails to be sent from Greenhouse Recruiting's dedicated Mailgun IP addresses which we actively monitor. The SPF record does not permit emails to be sent from all Mailgun IP addresses. You can find a list of Greenhouse Recruiting's dedicated IP addresses here.