In the final years before his death, Muhammad established communication with other leaders through letters, envoys, or by visiting them personally, such as at Ta'if, Muhammad intended to spread the message of Islam outside of Arabia. Instances of preserved written correspondence include letters to Heraclius, the Negus and Khosrau II, among other leaders. Although it is likely that Muhammad had initiated contact with other leaders within the Arabian Peninsula, some have questioned whether letters had been sent beyond these boundaries.