By default, many mail servers accept messages only from authorized mail sources. When you enable SPF records, recipients' email servers will check if your domain is listed in the SPF record. If so, the recipient's email server will accept incoming mail from your server. 

Creating SPF record for One.com

Here are the steps to set up SPF for One.com if your domain is hosted somewhere else:

• Login to your DNS management console 
• Navigate to Tools & Settings > DNS Template
• You need to edit the DNS TXT record related to SPF
• An SPF TXT record for One.com will have the following syntax: 

mydomain.com    TXT  v=spf1 include:_custspf.one.com -all

• Publish this record in your DNS records section for your respective domain. 

Note: If you don’t have an SPF record previously for your domain, simply publish the above-mentioned record to enable SPF for outgoing emails using One.com. If you do have an existing SPF record in your DNS, you need to modify it to include One.com, as shown below:

Previous SPF record:  v=spf1 include:spf.domain.com -all

Modified SPF record:  v=spf1 include:spf.domain.com include:_custspf.one.com -all

If your domain is hosted on One.com, 

• Log in to your One.com control panel and click on the Advanced settings > DNS settings
• Go to DNS records.
• Under create new record, click on TXT
• Copy and paste v=spf1 include:_custspf.one.com ~all in the “Value” section
• Save changes by clicking on “Create record”

Finally, you can look up your record using our SPF record lookup tool, and enable DMARC for your domains: take a DMARC trial.