Custom subject lines are reviewed by our Customer Solutions team.
We do require two things for this subject line to be accepted. First, you will need a call to action like "please confirm" or "verify that you would like more information..."
We also require a request for previous information. This is where you'll be reminding subscribers what they signed up to receive.
"Please, confirm your subscription to [your business name here]"
You can edit the confirmation message on the List Settings page.