In the Sahih-Bukhari, a generally accepted collection of the times and sayings of the Prophet, there are a number of reports of contacts with other nations & empires; for example:
Allah's Apostle wrote a letter to Caesar saying, "If you reject Islam, you will be responsible for the sins of the peasants (i.e. your people)."
Allah's Apostle sent his letter to Khusrau and ordered his messenger to hand it over to the Governor of Bahrain who was to hand it over to Khusrau. So, when Khusrau read the letter he tore it. Said bin Al-Musaiyab said, "The Prophet then invoked Allah to disperse them with full dispersion, (destroy them (i.e. Khusrau and his followers) severely)".
What independent corroboration of official contact between the Early Islam (the ummah) and the nations surrounding them is there during the Prophets time?