Mohamed, we are told, worked as a merchant prior to his “prophet hood”. To be involved in the business of selling, buying or trading one would have to be familiar with the numerical system. The Arabic numerical system didn’t come into existence until the 7th-8th century, after the time of Mohamed. Prior to its creation, Arabs used letters to symbolize numbers. Hence, Mohamed would have had to be familiar with Arabic letters.
In Q 2: 79, Allah warns the ummi not to write distorted versions of the scriptures with their own hands, proving that they have the ability to write. Hence ummi, as we can see, does not mean illiterate in the Koran. It means “A people without a Book” i.e. the Arabs.
In Iban ishaqs work (An 8th century historian) he recorded an instance in which Mohamed actually WROTE a letter to his contemporary who claimed to be a messenger of God. Reference: Guillaume, A., The Life of Muhammad, a translation of Ibn Ishaqs Sirat Rasul Allah, Oxford, 1967, p. 649. The Qur'an identifies Mohamed as al-nabī al-ummī (Q. 7:157–158)
Sahih al-Bukhari 4431 :
The ailment of Allah's Messenger became worse (on Thursday) and he said, fetch me something so that I may write to you something after which you will never go astray.
Mohamed sent letters to various individuals and leaders during his lifetime. Some notable recipients of his letters include:
1. Kings and Rulers: Prophet Muhammad sent letters to several rulers, inviting them to Islam and conveying the message of monotheism. These include the letters to Emperor Heraclius of the Byzantine Empire, the Persian ruler Khosrow II, the Negus (King) of Abyssinia, and other regional leaders.
2. Tribal Chiefs and Leaders: The Prophet sent letters to various tribal chiefs and leaders of neighboring tribes, inviting them to embrace Islam. These letters aimed to establish peaceful relations and spread the message of Islam in different regions.
3. Jewish and Christian Communities: Prophet Muhammad also corresponded with Jewish and Christian communities, engaging in dialogue and addressing specific issues. Notable examples include the letters to the Jewish communities in Khaybar, the Christians of Najran, and the monks of Saint Catherine's Monastery.
I am open to informed correction on any of the above as all sources regarding Mohamed are from 200+ years after his death!