Lightroom includes a preset that sizes and configures exported photos for email. You can edit this preset so that it opens your email software automatically when you export photos, allowing you to email photos from Lightroom with a single click.
Now that you have a preset that is connected to your email software, you can export photos to email in one click.
Note: If the Export dialog box was set to your emailing preset the last time you used it, you don't have to open it. Press Ctrl+Alt+Shift+E (Windows) or Command+Option+Shift+E (Mac OS) to bypass the Export dialog box and apply the last-used settings.