In the Outgoing Messages Queue are mails which are unable to be send, or you have received a non-delivery report regarding a sent mail.
The error message states that a PTR-Record (Pointer Record) is not found in the DNS, or that your mail server has a Dynamic IP Address.
In the DNS which is responsible for your IP Address (not the domain), enter the PTR Record.
It is required that you have a static IP address for this Action.
In most cases, this can be done via web-Formular.
You may get further information from your provider.
The name for this IP Address needs to be the same name as found in Configuration -> E-Mail Transport -> SMTP Settings -> Hostname (FQDN).
If no static address is available, your mail server needs to relay the mails over the Smart Host of your provider.
Check the configurations found under Configuration -> E-Mail Transport -> SMTP Settings -> Client Settings -> SMTP Relay.