Excel is an excellent way to input addresses and can easily upload into Outlook or other programs. I use
AWeber for my clients newsletters and emailings and you can input address from Excel and also export reports to AWeber. Many companies now limit how many emails you can send out before you are labeled as a spammer. Be very careful about sending emails for clients from your personal or professional email address. I now exclusively use a double opt-in for clients to protect myself. I maintain email lists ranging from 20 to hundreds. The cost for AWeber is minimal when it is divided between the clients that I support.