Although this is convenient, it is not always appropriate. If you are unsure, ask the sender before you forward a message. Email replies may go to more people than you realize. When replying to a message be sure to look at the list of recipients, especially addresses of mailing lists, which may redistribute your message to dozens or hundreds of individuals.
Email can be junk mail, so avoid unnecessary proliferation of messages.
Email takes up computer space, so delete messages you no longer need. The integrity of an email message cannot be guaranteed. If a received message seems out of character for the sender, double-check before taking it seriously. Email is meant for informal correspondence as well as scholarly and scientific communications. Email should not be considered private.