Sometimes to send emails on behalf of your domain, it being Google Apps, Microsoft365, Amazon SES, SendGrid or similar, you're required to add an additional "SPF Include" to your SPF.
This can be done in the "Email Deliverability" page in cPanel.
1: Go to Email Deliverability
2: Click Manage next to the domain you want to change
3: Wait for the interface to finish loading the current records and then click Customize under the SPF section.
4: Wait for the interface to finish loading the current records again and under the Additional Settings section, you can click on Add A New "+include" Item - here you can specify
_spf.google.com for example.
5: Click on Install A Customized SPF Record button.