Starting the list you already have.
THE SITUATION
You have customers, their addresses, and no list.
WHAT TO DO
Do not import them all and start sending campaigns. Some will not remember giving you the address, and complaints follow.
THE BETTER APPROACH
Send one message, personally written, explaining that you are starting a newsletter, what it will contain, how often, and inviting them to subscribe.
Include a subscribe link rather than adding them.
WHY THIS IS WORTH THE SLOWER START
You get a list of people who actively chose, which performs far better and protects your sending reputation.
WHO TO SEND IT TO
People with whom you have a genuine business relationship. Customers, enquirers, contacts.
Not addresses collected from anywhere else.
WHAT TO SAY
Short. Who you are, what you will send, how often, and a link.
A sentence on why it might be useful to them.
WHAT TO EXPECT
A fraction will subscribe. That fraction is your starting list, and it is worth more than the whole set would have been.
AFTERWARDS
Send the first proper issue within two weeks, while the decision is fresh.