Sending email used to be easy. In the past, even legitimate bulk email could be sent without problems.
Today, if you are sending bulk email, you should consider using double-opt-in customer lists. This is where the customer signs up for your newsletter, for example, but then receives a message asking them to verify that they wish to receive the newsletter. In this manner, the chances of a customer getting on a list that they don't want to be on is greatly reduced.
Why should you care? In addition to providing good customer service, you don't want to irritate a customer into reporting your domain/IP to a spam reporting service. If this happens, your future email may not reach many of your customers, as their email server may check the blacklists prior to allowing your email through to their account. In this case, you will need to spend time working to get off their blacklist. In some cases, you may even have to pay to get your domain/IP delisted. So it's always better to prevent blacklisting than to try to address it after it has already happened.
In an effort to combat spam, some services are now looking not just at the content of email, but also at the number of recipients. So in addition to using double-opt-in lists, it's best to limit the number of recipients to 10 or less. What if you have 1000 customers? Then you'll need a good mailing list program, which can send the email out in small groups over time. Note that this is a best practice anyway, as your server (especially a shared server), can get bogged down quickly if you try to send all your email at once.
Due to a common tendency to over-protect individual accounts from spam, you should also ask your customers to whitelist your domain. This prevents your legitimate email from getting filtered into a customer's spam-box by mistake. You may also be able to get your IP address whitelisted by some of the spam listing services, by proving your entity is reputable and only sends legitimate bulk email.