Muhammad (c. 570–632) is documented as having engaged as a diplomat during his propagation of Islam and leadership over the growing Muslim Ummah (community). He established a method of communication with other tribal or national leaders through letters, assigned envoys, or by visiting them personally, such as at Ta’if. Instances of written correspondence include letters to Heraclius, the Negus and Khosrau. 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.