The program blocks messages that are obviously spam without bothering to issue challenges.

From the toolbar the user is able to manually block or approve e-mail messages.

AT&T Wireless has developed a technology to block messages sent anonymously or to large blocks of customers.

When things become hectic, the system filters distractions, blocking calls and all warnings or other messages that are not critical for safety.

However, settings allowed any user to block messages from users if they choose.

This eliminates nearly all spam, but it often also blocks legitimate e-mail messages from senders who may not yet be in a user's address books.

Private messages are available, as well as the ability to block messages from people you don't like.

Many spam filters automatically block messages with subject lines that contain specific words.

Many filter programs also block messages with a large number of people in the To: field.

Some operators of commercial computer networks, meanwhile, have threatened to block anonymous messages of any sort.