Deskpro will reject emails sent from email servers that do not have a valid rDNS (reverse DNS) entry configured.
Email servers without a valid rDNS entry are abused by spammers to send unsolicited email, or by malicious actors sending phishing emails. Most ISPs (including most email providers like Gmail) also reject these types of emails.
When this matters
This is most likely to become a problem if you are running your own email servers. If you are running your own servers, there is a large list of potential issues and gotchas to do with "email hygiene" that you need to contend with, and a missing or invalid rDNS entry is just one. Other common issues include IP health/reputation, SPF/DKIM, and blocklists.
This might be most notable if you are using custom email addresses that forward into Deskpro (i.e. firstname.lastname@example.org -> support@YOUR_ACCOUNT.deskpro.com). Your end-users will usually send email via a well-behaving email server, but since all email flows through your custom address first, it is your custom email server that needs to be in good health because that is what is ultimately "checked".
Checking your rDNS
You can use a tool like MXToolbox to check your rDNS: https://mxtoolbox.com/ReverseLookup.aspx
How to configuring rDNS
rDNS is typically configured via your hosting provider. How you configure rDNS will therefore depend on your provider and you will need to consult the guides and manuals with your provider to learn how. It is usually a case of logging in to a control panel and finding the "rDNS" or "Reverse DNS" section.
You need to create a rDNS record such that the IP address of your server -> points to a hostname (e.g. my-email-servers.example.com) -> which in turn resolves back to the same IP address.