Are you posting this in support for the calendar extension because emails are not being sent when you sign up from the calendar pop-up sign up form?
Or are you signing up from the main plugin’s signup/list page? If it’s the main plugin, then you should post in the support forum for the main plugin at wordpress.org.
Sounds like you changed something which broke your emails, maybe some invalid content for a plain text email that’s causing WordPress to filter it out?
Not really enough info for me to figure out what is happening on your site. I just ran another test here and signed up from a calendar pop-up signup form, and also from the main plugin’s form, and got emails without any problems for both.
Maybe double-check your settings, and then talk to your hosting provider to check your server logs to see if there were any errors logged when WordPress was trying to send those emails.