Email dns settins help

Hi all I'm looking for some help. I have a dropbox in Rex where our emails are forwarded to, although the set up is right and I followed instructions. Emails being sent to us are not being forwarded to the dropbox email.

I got this from our CRM hosts

mail scanners/virus scanners/security settings may look at the DNS records configured on your domain and show an error or spam list the email.

This is typically a result of SPF DNS records on your domain being configured to allow sending emails for your domain ONLY from your servers - alternatively, it can also be the result of no SPF entries being configured for your domain at all.

Can anyone help asap? I don;t think its a big job so good quick help would be appreciated. Please only experienced people who know exactly what theyre doing.