At TWC we always mask email addresses when we send group emails by using a function called BCC (Blind Carbon Copy or Blind Courtesy Copy) and we encourage everyone who sends emails to multiple recipients to do that as well. Using Blind CC makes it all that more difficult for spammers to snake other people's email addresses. Peter Deegan at Email Essentials elaborates.
We’ve mentioned this before but we still get questions about the Myth of BCC – Blind Carbon Copy. This story alleges that spammers can grab your email address from the BCC list in a message, not just TO and CC. This is wrong – and it’s so easily checked that it’s amazing that the story persists. All you have to do is look at the header of an incoming message for a BCC list of addresses – you won’t find one. When you send a message with BCC’s, that list of addresses is stripped out by the sending SMTP server (most likely at your ISP) and BCC’s are sent to each person on the list.
And there is one more consideration. You wouldn't post a friend's cell phone number in public and you should afford them the same courtesy with respect to email addresses.
As Ever, TWC