The basic question here is : "Do we need to accept tickets from all email addresses ? "
It so happens that an organization chooses to accept emails from specific email addresses or filter out specific email addresses.
The reason could be security specific or process specific.
Spiraldesk provides a way to do this by white-listing or black-listing IP addresses or email addresses.
To work this out , go to Settings --> Helpdesk settings
Please make sure you enter the values as Comma-separated fields.