When you add an action button that generates an email from Slack, you want to make sure it opens with your preferred mail client.
To set your preferred client on desktop....
...for Gmail
Open Chrome Browser and click More Settings.
At the bottom of the page, click Advanced.
In the Privacy and security section, click the Right arrow next to Site Settings.
Towards the bottom, click Expand next to Additional permissions.
Click the Right arrow next to Handlers.
Turn on Allow sites to ask to become default handlers for protocols (recommended).
Open Gmail in your Chrome Browser.
In your browser’s address bar, click Service handler .
Click AllowDone.
...in Internet Explorer
Click here to download the Google Tooler for Internet Explorer.
Once installed, click the wrench icon at the top right corner of your Internet Explorer browser to open the Toolbar Options window.
Scroll down to the Web-browsing tools section.
Select Use Gmail for 'Mail To' links," and click Save. Now you're good to go!
... for Outlook
Open your Windows computer's Control Panel.
Click Internet Options.
Click the Programs tab.
Select the desired email client from the drop-down menu.
Click OK.
Once you do this, clicking email action buttons should open with your preferred mail client.