Letters and writings are enough proof on religious matters in the normal circumstances

اذْهَب بِّكِتَابِي هَـٰذَا (Go with this letter of mine - 28). Sayyidna Sulaiman (علیہ السلام) considered it enough to write to the queen of Saba' (Sheba) for the fulfillment of his duty to invite her to faith, and hence sent her a letter. This shows that in the normal circumstances the letter or writing is an acceptable proof. The religious jurists did not accept a letter as an evidence only where a proper personal evidence is required under Islamic law, because testimony is not allowed on telephone or by letter. It has been made mandatory for the witnesses to appear in person before the court. There is a lot of wisdom in this law. Under any law of the world prevalent in any country it is compulsory for the witnesses to appear before the court in person, and the testimony through letter or telephone is not accepted.

Writing letters and sending them to the disbelievers is permissible

Another matter that has been pro-ved by the letter of Sayyidna Sulaiman (علیہ السلام) is that in preaching of religion and invitation to Islam it is permissible to write letters to disbelievers and infidels. According to many Sahih ahadith it is established that the Holy Prophet ﷺ had also written letters to many infidels.

Social etiquettes should always be observed, no matter if it is a gathering of infidels

فَأَلْقِهْ إِلَيْهِمْ ثُمَّ تَوَلَّ عَنْهُمْ

Throw it down to them, then turn back from them - 27:28

When Sayyidna Sulaiman (علیہ السلام) made use of the hoopoe as a courier, he also taught him the social etiquette that after delivering the letter to Queen of Saba' it should move out of her court, which is the norm of the royal courts. This shows that observance of social etiquettes and good human behavior is desirable as a general rule.