SPF stands for Sender Policy Framework. In an SPF record you can define which servers can send emails on behalf of the domain name. A receiving server can, among other things, decide on the basis of the SPF record to let an email pass, mark it as unsafe or refuse it altogether.
We do use an SPF record to prevent our emails from being marked as spam.
The following TXT record can be added to the DNS of the customers:
v = spf1 ip4: 184.108.40.206/23 mx
v = spf1 ip4: 220.127.116.11/22mx