When you click on any e-mail address in the CRM, your default e-mail client (Outlook, Thunderbird, ...) will open automatically with a hidden copy of the e-mail assistant.
If you are using a system other than the default, you can change it in the operating system settings.
Instructions for Windows
Via the Start menu or the magnifying glass, find the default application settings, open the folder.
Select the email application you are using.
I use Gmail
To set Gmail as your default email client, you need to download a special application, an add-on that will allow this in windows. It's called Easy Mail for Gmail and it's available for free here: https://www.microsoft.com/en-us/store/p/easymail-for-gmail/9nblggh626nc
After installation, run the application.
Sign in with your Google Account. You currently have the add-on installed and need to set it as the default (see the procedure above).
Instructions for MacBooks
You can find it nicely laid out here: https://support.apple.com/cs-cz/HT201607
I use G-mail
To set up Gmail as your default email client, you'll need to download a special application, an add-on that will allow this on your Mac. It's called Kiwi for Gmail lite and it's available here and for free: https://itunes.apple.com/us/app/kiwi-for-gmail-lite/id1006184923?mt=12
After installation, follow the instructions above.
If you have set everything up according to the instructions, and yet the correct email window does not open after clicking on your email address, there may be a problem with your browser settings.
In Chrome, click Chrome Settings at the top right of the screen ► Sections in the left menu Privacy & Security ► Web Settings ► Utilities ► Check your email client. Once you have set up an email client here, there is nothing stopping you from sending emails directly from your CRM account.