If you are sending survey invitations using your own email address it would be the best to add both SPF and DKIM records to your domain name services (DNS). This will assure your emails will go through to your own domain email addresses.

SPF: Sending server IP validation. Used by majority of inbound mail servers. SPF Information.

DKIM: Like SPF, but uses cryptographic methods for validation. Supported by many inbound mail servers. DKIM Information

Add the following SPF as a TXT record to your DNS.

v=spf1 include:mailgun.org ~all

Add the following DKIM as a TXT record to your DNS using subdomain name: k1._domainkey

k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDutgkyyiKfcVfwizKmTF71EYhm0pr9rRw5PwDQv1h3ytLcbqX30V/IVtgALy1vOQXh6tpJCtbEwhwUZlfsw/3quLd6LQ2Px6PFgBrRsv+85PSPo0SdoTC8cSJKuWRNgh6N7qA+F2WGmOADu7DssysVHz5kAcuTdjHzPi5JHg/nYQIDAQAB

📘
More about SPF
http://www.openspf.org/Introduction

📘
More about DKIM
http://www.dkim.org/#introduction