The best tool for debugging eMails locally in my opinion is Mailcatcher. It catches all mails being sent via a specific port and shows them to you via an neat interface in your webbrowser. Firstly install Mailcatcher. It's programmed in Ruby and I could install it easily on Ubuntu and Mac. The instructions can be found at https://mailcatcher.me/.
In the full stack edition it comes with symfony/swiftmailer-bundle. Make sure these settings are in your parameters.yml + parameters.yml.dist file.
Afterwards you can send a test mail via the console:
The mails will be shown to you like in the screenshot below.
Beside Drupal 8 install the "SMTP Authentication Support". You can find it's project webpage at https://www.drupal.org/project/smtp. Then visit the following page in your Drupal 8 instance: http://your-neat-webpage.dev/admin/config/system/smtp. Make sure the following settings are set in the config form (there's also a possibility to set them in the settings.php file).
SMTP server: 127.0.0.1
SMTP backup server: LEAVE BLANK
SMTP port: 1025
Use encrypted protocol: No
SMTP AUTHENTICATION: LEAVE USERNAME + PASSWORD BLANK
E-MAIL OPTIONS: ANY EMAIL AND FROM NAME
SEND TEST E-MAIL: ANY E-MAIL ADDRESS
Check "Enable debugging".