Thanks for the suggestion, but I think you misunderstood the issue I encountered with. I can save the plugin's settings in the WordPress admin area, but when I test the plugin, it still sends the confirmation email with the previous sender. I can see it only when I check the test email in my email client, in the WP admin everything seems to be fine.
What I recognized: the plugin uses the "Site Title" saved in the Settings/General WP Core setting page instead of the option which is given in the plugin's settings. When I change the Site Title, the plugin sends the email with the same sender name I set as the Site Title.