This alert is entirely unique to your organization's mail network.  We suggest you provide the following info from our Email Resource FAQ to your email network administrator:

At times, this alert can be resolved by adding an SPF Record (Sender Policy Framework) entry to your DNS (Domain Name System) records. Additionally, having an SPF record set up on your domain can help to prevent emails from being identified as SPAM.

Configuring DNS records are not supported by Blackbaud, as different providers have unique means in updating DNS records.

Please contact the appropriate software vendor or IT professional for assistance with this process or issue, which is beyond Blackbaud's scope of support.