If a mail bomb is sent to a user in Valise, all users in Valise will suffer. If a mail bomb is sent to a user with an ISP, all users with that ISP will suffer.
Mail bombs come in many different forms.
Either it is a very large mail or thousands of small mail. Usually if you have offended an impatient net nerd somehow, he will use an automatic program to send you thousands of letters to flood your mailbox as a revenge. Another form of mail bomb is from unauthorized newsgroup or list server subscription. A sexually frustrated net rat can simply sign on Valise and then spent a whole night subscribing to hundreds of newsgroups. Then this rat will drop dead and disappear from Valise forever.