If you are using one of our sign up forms on your website, then any new contacts that are added to your account automatically get the welcome email associated with your account. This is sent out immediately once someone signs up. For more information on the welcome email, take a look here.
If you wanted to set up an automatic email to all contacts (existing and new) that sign up for your list, then you would need to use an automated email to go to your contacts that are added to a specific list. Here's some information on that.
I hope that this helps. If you need any more information or have any questions, let us know.
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