If your email provider bounced back emails or responds with a permanent failure code then the 3cx SMTP server will place that email address on an internal block list. No more emails to you will be attempted. You likely need to contact 3cx support to have them manually remove your email address from their block list.
Before doing that, go through your mail settings to be sure you have not accidently put them into a blocked sender configuration in case it triggers a bounc back again immediately after they re-enable you.
If you have another email address, you can change the email for your extension to start getting notifications again immediately.
Also never setup an extension with an email you have not yet configured or just setup and may not be working yet on the back end. Make sure the email is working and valid first before configuring a new user extension and sending the welcome email.