The Sender Policy Framework, also known as SPF, is an email validation system designed to prevent spam email by verifying the sender's IP address. SPF allows administrators to specify which hosts are allowed to send email from a given domain, by creating a list of authorized hosts in the Domain Name System (DNS) records. Servers use the DNS to check that email from a given domain is being sent by a host who has permission to do so by the domain's administrators.
What this all means is that you can request that your site administrator creates a record in your DNS to mark Moosend's mail servers as valid sending servers for your domain. It only takes a few seconds and will validate your campaigns, which makes it easier for you to reach inboxes!
It's true, setting up SPF greatly influences your deliverability. Many email clients (e.g. Gmail, Verizon and others) use SPF to validate that the IP address the email was sent from is trustworthy. Think of it like a credit card authentication right before you make a purchase.
Moosend gives you the option to set SPF on your sender accounts, but in order to do that you must have control over the DNS settings of your sending domain. Just ask that your administrator sets your SPF by following the process described below:
1. Click on the Accounts button, located on the top right side of your Moosend account. 
2. Click on the Senders button, on the menu that appears. 
3. Check which of your sender accounts has a valid SPF signature. 
Ticked boxes under the SPF / DKIM column indicate sender accounts with valid SPF and DKIM signatures respectively.
4. Click on the sender email address which you want to set up. 
5. Choose the Sender Policy Framework (SPF) tab. 
6. Copy & paste the necessary information to your domain's DNS records. 
Note that depending on your hosting provider you may need to make small changes to the provided records.
Furthermore, it is important to remember that at this point and in order to finalize the process, it might be necessary that you wait anywhere between 30 minutes to 48 hours before you can move to the next step. After the waiting time is over, visit the page once again and proceed with the final step below.
7. Come back to your Moosend account, and click on the Verify DNS records button to complete the verification.
If you'd like to know more about SPF, you can start by looking at this interesting SPF article from Wikipedia.