For example, in Sahih Bukhari we have the following two reports:
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)".
Is there any independent corroboration about such letters; that is of official contact by the the early ummah and the surrounding nations?