Hey plc,
Yes if you use support@domain.com and configure OSTicket to check that address it will pull every email and try to create or update a ticket with it. There are a couple "solutions" for this.
1. You could stop using the email address personally and inform your customers that any email sent to that address will on DATE start creating tickets for them, and that if they want to contact you personally to use NEWADDRESS@domain.com.
2. You can setup a new email address specifically for them to open tickets by emailing. (say tickets@domain.com)
3. You can not setup cron/task scheduler and manually delete all the emails that you do not want tickets created for and then log into OSTicket with the Admin Panel -> Settings -> Enable Auto Cron enabled. Which would trigger OSTicket to check for emails to make into tickets. I feel that this is a horrible idea, especially if you have more then one staff handling tickets.
4. You could setup an email filter that sorts your email into multiple accounts, and then have OSTicket setup to check the second box. I dont know if this could be easily done in Windows, but in Linux I imagine that procmail would work and could be setup to filter on specific keywords. Check out http://www.procmail.org(http://www.procmail.org) for more information on that. I haven't used it in years and am not 100% sure that it would work the way I'm thinking it would.
Personally I feel that option 1 or 2 are the most viable, but ultimately you will have to decide what is the best fit for you and your customers.