Is there a way to do this without setting up an email account with mail first?

martinalex wrote:

Is there a way to change the default email client without setting up apples own email client first? I don't want to use it, so why force me to set it up in the first place?

Can you not make the change in Mail>Settings>General...(?)

Is your alternative email client installed?

ref: Change the default web browser or email app on Mac

Would follow the previous advise First..

If you have an e-mail files outside to any e-mail client >> Control click the file.

A pop up will appear " Get Info " - images below

Choose Open With and change it to the Third Party e-mail client installed on this computer

Note - Use the Change ALL.

This will make the Third Party e-mail Client the default application to open these File types.

It will not change the default Application the Default e-mail client

That would have be done by Original Reply posting by @leroydouglas above