The scribes of the Prophet (peace and blessings of Allaah be upon him)
Question

Who were the scribes who wrote the letters of the Messenger (peace and blessings of Allah be upon him)?.
Praise be to Allah.Ibn al-Qayyim said: 
The scholars of seerah (Prophet’s biography) have mentioned the names of the Sahaabah who used to write down the wahy (revelation) or the letters of the Messenger (peace and blessings of Allah be upon him). They were: Abu Bakr al-Siddeeq, ‘Umar ibn al-Khattaab, ‘Uthmaan ibn ‘Affaan, ‘Ali ibn Abi Taalib, al-Zubayr ibn al-‘Awwaam, ‘Aamir ibn Fuhayrah,  ‘Amr ibn al-‘Aas, Ubayy ibn Ka’b, ‘Abd-Allah ibn al-Arqam, Thaabit ibn Qays ibn Shammaas, Hanzalah ibn al-Rabee’ al-Usaydi, al-Mugheerah ibn Shu’bah, ‘Abd-Allah ibn Rawaahah, Khaalid ibn al-Waleed, Khaalid ibn Sa’eed ibn al-‘Aas (whom it was said was the first one to write down anything for him), Mu’aawiyah ibn Abi Sufyaan and Zayd ibn Thaabit. He requested them to do that and allocated this task to them. 
Zaad al-Ma’aad, 1/117 
Ibn Muflih al-Hanbali said: 
A group of them wrote things down for the Messenger of Allah (peace and blessings of Allah be upon him), including Ubayy ibn Ka’b, Zayd ibn Thaabit, ‘Ali, ‘Uthmaan, Hanzalah al-Asadi, Mu’aawiyah, and ‘Abd-Allah ibn al-Arqam, who was his regular scribe in charge of writing and responding to letters. He was the one who wrote down all the Revelation and whom the Messenger of Allah (peace and blessings of Allah be upon him) commanded to learn how to write Syriac so that he could respond on his behalf to those who wrote to him in that language. So he learned it in eighteen days. 
Al-Adaab al-Shar’iyyah, 2/161 
And Ibn Hajar said: 
Al-Qadaa’i said: Zayd ibn Thaabit used to write to the kings on his behalf, as well as writing down the revelation. And al-Zubayr and Jahm used to write down the records of zakaah. 
Al-Talkhees al-Habeer, 4/346, 347 
And Allah knows best.