I always use double opt in, but even then I'll send out a confirmation email. Someone who's just signed up for your emails may still be unsure of whether to trust you, unfamiliar with your brand, etc. It's a great opportunity to make a good impression.

Here are just a few things you might want to include:

• A bit about who you are
• What they should expect to receive in your emails
• Reassurance that you respect their privacy (links to privacy statement, etc)
• An to whitelist your address (+ link to explanation on how to do that)
  
• Instructions on how to unsubscribe if they ever need to

Minimising the impact of malicious signups
If you're not double opting in recipients (requiring them to activate their sign-up via a link in an email), it's worth remembering that people will be able to sign up other people (ie. your recipients may have been maliciously signed up). You may want to include a prominent unsubscribe/explanation for these people - eg "You have received this email because email@address.com signed up to this list. If you believe this to be an error please unsubscribe now."

I hope that's of some use.

daniel