What is an email gateway?
The email gateway is an email box that is used for storing and transferring email messages between the Help Desk Ultimate extension and your customers.
How does Help Desk Ultimate parse email messages into tickets?
After an email message arrives at the gateway mailbox (POP3 or IMAP), Help Desk Ultimate collects it from this email gateway after a cronjob launch; then, it is parsed into a ticket, which can be viewed and replied to from Magento itself (either from the backend, or from the Help Desk tab in a frontend account dashboard).
Is a gateway required for the extension to work?
No, setting up a gateway is optional. If you don't create any gateways, the tickets can still be submitted and processed, but via the webforms only.