In what year did the Messenger, may God's prayers and peace be upon him, write to kings inviting them to Islam?

The Prophet, peace and blessings be upon him, sent messengers to kings inviting them to Islam after the Treaty of Hudaybiyyah. Some scholars believe this occurred in late 6 AH, while others say early 7 AH. Both statements can be reconciled by understanding that some messengers were dispatched in late 6 AH and arrived at their destinations in early 7 AH, while others were sent in early 7 AH. This happened before the conquest of Mecca, and the sending of messengers continued until his death, peace and blessings be upon him.