How will he who has not received the message from humans be held accountable? And are Muslims to be blamed for their shortcomings in conveying it?

It is unlikely that there are peoples to whom the message of Muhammad, peace and blessings be upon him, has not reached. If such peoples exist, they will not be punished until proof has been established against them, as Allah Almighty says: "And never would We punish until We have sent a messenger." The ruling concerning these individuals is similar to that of the people of the Fatrah (interval between prophets), and they will be tested on the Day of Resurrection, as stated in the Hadith (definition); if they obey, they will enter Paradise. Muslims are held accountable if they fall short in conveying the message to those whom it has not reached.