It would be a great help if swpsmtp_init_smtp() contained a simple conditional at the top of the function to check whether any SMTP settings have even been entered before making changes to $phpmailer. As it is now, wp_mail() fails as soon as the plugin has been activated.
Even something as simple as strlen($swpsmtp_options['smtp_settings']['host']) && $swpsmtp_options['smtp_settings']['host'] !== 'smtp.example.com' would do the trick. That will at least allow wp_mail() to work the usual way until a user has gotten around to configuring the plugin.
This would be particularly beneficial in cases where the person installing the plugin is not the same person with the SMTP server details, or for sites that have security notifications emailed on certain actions (like changes to plugins, logins, etc.). :)
Thanks!