The letter was an unsealed diplomatic communication. This was an insult to the recipient because the courier was free to read it and spread its contents among the people of the region.
This means he had the letter in his possession, but he did not necessarily carry it in his hand at all times. AT: "in his possession" (See: [[rc://en/ta/man/translate/figs-metonymy]])
This means that they are planning to rebel against Artaxerxes, the Persian king, who was currently ruling the Jews. AT: "are planning to rebel against Artaxerxes" (See: [[rc://en/ta/man/translate/figs-explicit]])