Signature stamps work and are perfectly legal when sending a mass of business letters. These stamps may also be notarized. Legally, the purpose of a signature is to show intent and exercise of discretion in marking the paper; this can be done with a signature or a stamp.
The LivingWIthDisability website reports that in addition to business uses many people living with disabilities achieve increased functionality and independence though the use of signature stamps. The use of signature stamps and other mechanized means of reproducing signatures is common in many public and private sector applications. There are countless documents signed with signature stamps every day.