It said it would keep two other Italians and one Lebanese who were kidnapped during the same raid.
The group said: "There are no discussions ongoing with regard to the release of the remaining two Italians and one Lebanese still in our custody. They are being held indefinitely."
All four of the hostages worked for Agip, a unit of Italian energy firm Eni.
In Rome, the Italian Foreign Ministry confirmed Dieghi had been freed and Italian state television quoted an Eni source as saying Dieghi was in a good condition.
Dieghi's wife was reported on Italian television as saying she had spoken briefly to her husband by satellite phone.
MEND said in its email: "The eventual release of all hostages in our custody and the possible release of all hostages of Niger Delta origin in Nigerian government hands will not signify an end to our campaign.
"We will rather change tactics, desisting from kidnappings and concentrate on acts of sabotage, including bombings, aimed at crippling the oil sector."