It is possible to add mails to tasks automatically by mentioning the task number in the subject field (eg "#34789").
For situations where the subject cannot be modified (e.g. filling out a contact form on a third party site), it would be practical to add the task id to the helpdesk mailbox address: So if the helpdesk mailbox address is "email@example.com", sending mail to "firstname.lastname@example.org" would automatically attach the mail to task #34789.
Email Adresses with a plus-sign ("+") are valid as per RFC 2822 and this functionality is also used by other services, eg. Pipedrive.