Single opt-in means, subscriber gets message series without extra confirmation. (In contrast in double opt-in, subscriber clicks on a link to confirm his/her consent to receive messages from you)

Single opt-in has it's own dis/advantages.

Advantages:

* No hassle to the subscriber.
* No need to send another email from the software.
* Sign-ups will be more.

Disadvantages:

* Your list will be having more free email, junk email addresses. (Big no - no)
* You don't have the actual confirmation from the subscriber, it is not a permission based marketing.
* Anybody can insert anybody's email address in to your form and you don't know it > ending up in spam complaints.

UNLESS YOU ARE SURE ABOUT THE SUBSCRIBER, DON'T use single opt-ins. Like you are asking your present clients to sign up in their members area.