I'm no Don Laprie to suggest using a client mailing list to send occasional updates. When you have a new announcement, everything is already in place. If you have client-management database software, it likely already serves this function. If not, these are a few ways to send an email blast. Just remember that spam is as much about context as about multiple recipients; send email announcements sparingly.
If you're the only person sending to an email list, you can manage the process on your PC. In Outlook, choose File, New, Distribution List. Click Select Member to choose from your existing address book, or pick Add New to enter new contacts. Add the people needed. Type in a name, and click Save & Close. When sending a new message, click To, select the list, and add it to the mail. Triple-check all of the details before shipping it out.
To communicate in small groups, such as within your company, try a server-hosted mailing list. You could set these up for announcements or even have opt-in, casual lists for intra-office jokes.
These kinds of lists are managed on your mail host. The creation and configuration process will vary depending on your server configuration. But usually, you'll log in, open a mailing list preference area, name a new list—the email address people will send to—and let people sign up. (They'll send a message to join.)
Pay specific attention to a few options. Set the replies to be personally directed to an email sender to prevent lots of "me too" junk from being sent to everyone. Include a subject prefix to help sort messages in email clients. And allow only list members to send messages to prevent outside spam making it in.