Hi, nice to meet you! Here are some answers, let me know if you have any follow-up questions.
1.) The API allows you to set which list you are adding a contact to, as well as who added them. If you set it as an ACTION_BY_CUSTOMER (customer in this case would be you), the API will not send an automated welcome email. If it is set to ACTION_BY_CONTACT, a welcome email will be sent automatically. This welcome email can be set up and customized by logging in to the UI.
2.) You will need to add the contact to a list first, then send them to the Do Not Mail. Like I mentioned in the last point, you can use ACTION_BY_CUSTOMER to avoid sending out any sort of emails, then immediately send them to the do not mail list by sending a DELETE request to the contact's ID.
3.) The bulk uploader will always import as ACTION_BY_CUSTOMER, and when contacts are added this way, do not mail contacts cannot be put back into an active status. Once someone is on the do not mail list, it requires an action by contact to opt themselves back in.
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