(Go you with this letter of mine and deliver it to them then draw back from them and see what they return.) Sulayman wrote a letter to Bilqis and her people and gave it to the hoopoe to deliver. It was said that he carried it on his wings, as is the way with birds, or that he carried it in his beak. He went to their land and found the palace of Bilqis, then he went to her private chambers and threw the letter through a small window, then he stepped to one side out of good manners. Bilqis was amazed and confused when she saw that, then she went and picked up the letter, opened its seal and read it. The letter said:
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.
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 26–34
The Sabeans were a wealthy community of ancient times (1100 B.C. to 115B.C.). Their country was located in Ma‘arib (Yemen), where its grand ruins are still in existence. During Solomon’s period, this area was under the rule of a queen called Sheba (Bilqis). The people of this place used to worship the sun, Satan having taught them that the only thing worth worshipping was whatever was most prominent. As the sun was the most prominent of all visible things, only the sun, therefore, deserved to be considered a god and worshipped. Solomon received detailed information about the Sabeans through the hoopoe bird. This hoopoe perhaps belonged to Solomon’s army of birds and possibly received regular training.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.