Yes, for example GMail with Thunderbird or iCloud with the native mail app, probably some people just don’t know that they can connect their Outlook mail to another mail client.
Unfortunately Google is increasingly making it harder to use gmail with a native mail client. They don’t use normal means of authentication so your client has to have built-in Google support. They’re introducing new types of emails that can only be opened from a web browser or Google’s proprietary email client. I won’t be surprised if I wake up one morning to see that gmail simply doesn’t work with normal mail clients anymore.
