@saltimbanqui 

Is “DonDomino” an emailprovider or merely your private/business domain ? Are you unable to send & receive or only unable to send email ? Do you use a KPN connection at your home to connect to "dondomino” ? Or do only you have a KPN mobile subscription ? Or both ? Or do you have neither but problems when you try to send(/receive) email to/from a KPN emailadress ?

It is an email server, I cannot send or receive from my home network or my mobile network, which is also KPN's. If I connect to other Google or iCloud servers there is no problem

@saltimbanqui

Do you use a VPN ? That's a very good recipe to get blocked by the KPN.

Do you receive an (error) message when you try to connect and which message ?

@saltimbanqui Try to set firewall level to low. Most common ports for pop3 and smtp are blocked by KPN firewall

Thank you for your reply. I don't use VPN. I have already looked at the firewall and set it to low, but I still have the problem.

Which Experia/KPN Box do you have and which version of the firmware is installed?

What is the url of the imap/pop3 and smtp server of dondominio.com?

Which mail cliënt do you use?

I have the feeling that the subdomains for imap/pop3 and smtp are not resolved by the DNS servers of KPN.

Hello @saltimbanqui, if possible could you answer the question asked by @wjb With the answers we can probably get a little closer to the solution. 

Thank you all for your responses. I called KPN and they couldn't give me a solution, so I changed the mail server and now everything is working fine.
Best regards.