Holders of the postal receipt for the residence permit renewal can leave and enter Italy, only provided that:
· they have the passport, the postal receipt of the residence permit renewal and the original of the expiring/expired residence permit;
· the travel does not include transit through Schengen Countries.
Also in this case, for travelling in the Schengen Area and in other countries, you are strongly recommended to verify with the embassy/consulate of the country you are planning to go to, if you need a visa for entering, according to your citizenship and your current domicile.
Before starting a journey and buying a ticket, it is in any case advisable to ask to the Immigration Desk at the International Affairs Department.