This is due to a configuration problem on your Linux computer.
You must set your email software to launch when you click on a link to send an email:
on your Linux desktop, click Applications -> System Tools -> System Settings
click on the Details icon
On the left, click on "Default Applications", then, on the right, scroll down the "Emails" list. Then select your favorite email software.
From this moment, if, from Simple CRM, you click on a link to send an email, it is your usual email software that appears.