Interesting topic, you almost have to let HMP reopen the job becuase someone might say "Thanks, but now I wan't something else", which might be a new job, so maybe you'd close the original and create a new one, etc etc. So you can't use exclude emails containing thanks, thank you, etc.
How about a nifty system tag 'Thanks' that you can insert in an e-mail template.. This creates something people can click (hyperlink, button?) and it will create a 'Thankyou note' in a new area of HMP, called the "Satisfaction Centre". This might have reporting around it so helpdesk managers can use it to rate service.
Additionally there might be an out-of-the box about rating service with buttons (eg below) with a space for comments. This would be added to e-mail templates, or on the web module. Responses would be tallied in the satisfaction centre in summary with links to the jobs created, there would be reports giving averages of service by staff/department/job type/etc.
o satisfied
o dissapointed
o very dissapointed
o not interested