(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:
She (the queen) said (to her officials after receiving the letter), "0 chieftains, there has been thrown to me an esteemed letter, 27:29.
Literal meaning of Karim is respectable, honorable or esteemed, and idiomatically this word is used for a letter when it is sealed. This is why Sayyidna Ibn ` Abbas ؓ ، Qatadah, Zohair رحمۃ اللہ علیہما etc. have interpreted the expression کِتَابپ کَرِیم as the sealed book, which indicates that Sayyidna Sulaiman (علیہ السلام) had put his seal on the letter. When the Holy Prophet ﷺ learnt about the tradition of the non-Arab kings that they do not read the letter if it is not sealed, he got a seal made for himself, and used it on the letters he sent to Caesar and Chosro. It shows that the practice of sealing a letter is to show respect to the letter as well as to the addressee. The present day practice is to secure the letter in an envelope, which is as good as sealing it. Where show of respect to the addressee is intended, it is closer to the practice of the Holy Prophet ﷺ to secure it in an envelope.
The language of the letter of Sulaiman
Although Sayyidna Sulaiman (علیہ السلام) was not an Arab, yet it is not unlikely that he knew the Arabic language, especially when he knew the languages of the animals. As the Arabic language is supreme among all the languages, it is possible that he might have written the letter in Arabic. It is also probable because Bilqis was an Arab by race, and she read the letter and understood it. The other probability is that he had written the letter in his own language and an interpreter read it for her. (Ruh)
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.