Here are some questions you have to ask before re-using materials that have been sent to you by e-mail:

1. Is the message being sent to you as a personal message? If so, you will in any case have to ask the sender's permission to use it elsewhere, since it is as protected by copyright as a personal letter.
   
   
2. If it is a personal message, there is also the issue of privacy to take into consideration - you might be violating the sender's right to privacy. Another reason to ask permission.
   
   
3. If you are receiving the information on a list (such as EASE), the position may be safer, since the work has already been "broadcast" or "communicated to the public" (most list archives are searchable on the Internet), but I would still make the effort formally to clear the rights before republishing the work in a commercial publication of any kind.
   
   
4. Has the sender asserted that the contents of the message (joke, article, etc.) are his/her own work? If not, just because someone else has done the original "borrowing" of copyright material does not save you from the charge of abetting the offence by disseminating it further.
   
   
5. If the sender is simply re-sending material picked up elsewhere, you should be very careful. The materials may have been sent to you as a part of a personal communication, but it is quite a different thing if you, in turn, send it further to a list, or onto a website or re-publish it elsewhere. You have no certainty that the sender has cleared the rights for such use, and you should therefore either try to clear them yourself, or ask the sender to secure the clearance.

There is actually quite a body of law that applies to the Internet. Don't believe that you are immune out there or that "it is still vague".

Thanks to Christopher Zielinski of the UK for these comments.