This may seem like an obvious blog post to most readers. If that's the case, just move along, nothing to see here. However, if you're wondering why people have been giving you snappy responses and a stern look when you include them on mass emails where all recipients are in the "To:" field, please take a seat. I'll explain why you'll find yourself on the naughty step if you do it again.
First, when emailing dozens of people at once, you're sharing the recipients' email addresses with everyone else. Everyone, including aunt Wendy whose old computer is crawling with malware and nasty things that can harvest their address book. And including that careless salesperson who is quite happy to add me to their spammy mailing list even though I've never agreed to it. If you think that's acceptable behaviour, then you should have no problem with me taking your personal mobile number and plastering it all over the city, right?
And secondly, in particular when you're in a business environment, it looks awfully unprofessional to email customers or prospects openly. If you're sharing a prospect's information with no regards for their privacy, why should they trust you with information like credit card details? You've lost a sale right there.
Do yourself a favour and learn to use the BCC (Blind Carbon Copy) field of your email client when sending group emails. Even better, use proper email marketing software (Campaign Monitor, MailChimp and many more) and present yourself like a real professional.
Note: I've created this post to ensure I can send this link to email marketers and friends who don't understand why it's inappropriate to send mass emails this way. Feel free to link to this post if you also need to explain it to someone.