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
More about DKIM