How are non-machinable letters processed?

Non-machinable letters require special handling because they do not meet certain criteria necessary for automated processing. These criteria can include size, shape, and weight restrictions. Because they cannot be sorted automatically by machines, these letters are processed manually. Manual processing involves postal workers physically inspecting and sorting these letters to ensure they are delivered accurately and efficiently.

The other options might suggest forms of processing that apply to standard mail, which can be automated and handled digitally. However, non-machinable letters specifically necessitate human intervention, making manual processing the correct choice.