SubsiMail doesn't send mail “on its own” from nowhere — it needs an SMTP connection to actually deliver mail, same as any email tool. Your options are: connect a regular mailbox (Gmail, Outlook, etc.) for lower volume, or connect a dedicated SMTP relay/transactional provider (SendGrid, Postmark, Amazon SES, etc.) for higher volume or more sending control. For getting started and testing, a regular mailbox is simplest. Once you're sending consistently in the thousands per month, a dedicated relay service usually makes more sense for deliverability and sending limits.