It could be any of a dozen of different things. Ranging from your PHP config, to your osticket settings, to mail server rejecting it. You have not really provided us with enough information to really assist you in troubleshooting this. Are you using PHP email? SMTP? etc.
admin panel -> settings
scroll down to Email Settings.
What do you have Outgoing Emails set to?
What do you have System Email set to?
What to you have Default Alert Email set to?
scroll down to Alerts & Notices
What do you have enabled here?
admin panel -> emails
click on your default alert email
if you are using SMTP are your settings here correct?
Is the email you are using to open a ticket the same as your default email address?