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 Account button, located on the left side of your Control Panel page.
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 want to set up.
5. Click on the Sender Policy Framework (SPF) tab.
6. Copy & paste the necessary information where applicable, to verify your SPF records.
7. Click on the Verify DNS records button to complete the verification as soon as you're done.
If you'd like to know more about SPF, you can start by looking at this interesting SPF article from Wikipedia.