I would like to verify whether or not the removal of unsubscribed recipients takes place synchronously at the time of sending or not.

By this I mean, when I send a new mail to say, 500 recipients, does the system synchronously remove unsubscribed recipients up front as part of the send button click, or are unsubscribes handled asynchronously?  I am guessing the former, but need to verify.

For more details on why I need to know this.  We are using an "opt-in" system wherein recipients will be sent an initial mailing asking them if they want to be included in future emails.  If they take no action then they will not receive any future emails (will be unsubscribed).  If they wish to receive future emails then they need to navigate to our custom unsubscribe page and "subscribe" to future emails.  To accomplish this, we send the initial email to a batch of recipients and then immediately unsubscribe them.

So, in essence I need to know if I can go in and unsubscribe the recipients right after sending the new mailing, or if I would need to wait for the mails to be physically sent.  I don't want to kick-off a mailing and then unsubscribe some recipients before the initial mailing is sent.

Thank you!