Hi Bendik,
just go this steps this may be helpful to you how to achieve this requirement..
go to spro->crm->E-Mail Response Management System->Define Repository->select ERMS double click on Action Parameters there you will see action id AUTOACKN here find out the action service id AH_AUTO_ACK.
go back to spro->crm->E-Mail Response Management System->Define Services find out the AH_AUTO_ACK. double click on that there is clas CL_CRM_ERMS_AH_AUTO_ACKNOW this will trigger in standard when ever mail comes from outside to crm this will generate the Acknowledgement mail to the respect mail just debug that class you will come to know what is happening over there.
if you want to know which class is getting trigger you have to know the rule policy.
go to web ui click on servicepro.
click on service operations-> in search workcenter there is rule policies click on that search the policies.
select service ticket related one or do you know which rule ur using in your service module click on that there is a released rules under click on rule condition.
you can create your own Acknowledgement also and pass that custom acknowledgement to action in rule.
Thanks & Regards,
Srinivas.