This is a good time to talk about the CAN-SPAM act and the rules you must follow, particularly if emailing customers in the USA. Here is a basic outline of the CAN-SPAM guidelines:

1. You must include an unsubscribe link in your marketing emails. You must honour all unsubscribe requests within 10 days (ideally immediately).
2. From addresses and names and subject lines must be relevant (not misleading). 'Unsolicited' (bulk marketing emails) must include a legitimate physical address of the sender.
3. You cannot email harvested addresses or include false headers in your emails (technical stuff). Ask permission, basically!

The penalties for not abiding by these rules are quite severe and can include fines of up to $XX,000. It is for good reason that these laws exist: without them there would be absolute chaos. I'm sure we've all experienced the entirely unsolicited, evidently bulk email that asks us to register for something or purchase something else. It's not pleasant - the CAN-SPAM rules are designed to stop this from happening.

Including a good clear way for your customers to opt-out with one click is the best approach, along with detailing the name of your company and a short description of why they're receiving this email.

If you want to emulate the best then consider automatically unsubscribing your customers after a certain period of activity. I'm not talking about a mere few days but tracking if a customer has not opened or clicked an email for 30 days is a good way to guage that their interest levels may have waned. This is particularly important if you send promotion-heavy content such as daily newsletters.

Here's a great example from Fab.com. They use the 'automatic unsubscribe' as an opportunity to 'clean' their list and, in fact, reengage those customers that do click through on this final email: