Hi Roger, if customers have signed up for your mailing list, you should be all set.
When you say clientbase, what do you mean?
Is this customers who have created accounts, but not signed up for your list?
Is it all customers who have placed orders, but not signed up for your list?
Either one of these last two scenarios, really, would be unethical where you did not get permission from the customer.
I know many companies care less about ethics when it comes to email, but regardless, if you wanted to add those to your email list, it would require some development work to extract the email addresses and add them to your mailing list.
There is not built in feature to do that.
I recommend you add your signup tick box to checkout (if not already) and use double opt in to be sure you have a clean list.
If you are sending emails to a list that folks did not opt into, you need to be prepared to have that email address blacklisted with some email providers... Yahoo will do this (along with aol and frontier who are part of yahoo) and live, hotmail, outlook (Microsoft's related emails)... It can take weeks to get off that blacklist.
I've been doing this a long time and found out some of these lessons the hard way.
One of my stores has 150,000 email subscribers.
https://www.ecommercetemplates.com/help/ecommplus/parameters.asp#collectemailI recommend DO NOT use this noconfirmationemail=TRUE - if you have it in includes, I suggest you remove it so the user can confirm they wanted to sign up for your email list.
Thanks,
David