You would add the below TXT record to your DNS to prevent the email spoof error if you did not purchase a custom URL.
TXT @ v=spf1 mx a a:dedrelay.secureserver.net -all
Note that you can only have one SPF record in a DNS file, so if you already have one, you need to combine them.
combining mail chimp and grow labs SPF into one SPF
v=spf1 include:servers.mcsv.net include:mailgun.org include:spf.mailjet.com include:servers.mcsv.net ?all