HI Giulia,
If you copied same standard workflow 38 means.
After Release at approver level,You will get Release notification at Workflow initiator SAP inbox only.(Not in SOST because this place for External outlook mails)
If you want notify a person thru external mail means,You have to add send mail step.(Red pointed place you have to replace with send mail step)
Hope So Dont change the Release requisition Task TS00007986.(Use the same standard Task otherwise teriminating event will not capture properly it leads to Workflow hang up)
Regards,
Ragav