Hello @EileenM514 ,
So we do have some articles that might be able to shed some light for your contacts. With that said, much of a contact's deliverability issues can be tied to their email provider, or the various security systems that they may or may not have access to between their ISP, network, and business/organization. The things a contact will be best able to control are their automatic folder assignments (e.g. Gmail's Promotions or Office 365's Other folders), and safelisting your email address in their email client.
If they have access to their own security, or easy access to their org's IT team that manages that, then they can also look into safelisting our domains (and other 3rd party domains we utilize). This could also apply to any pop-up / ad blockers, VPNs, or security applications that block may block our links or cause issues when viewing images.
The main articles to reference for your contacts: