What’s the difference between single opt-in and double opt-in?

Single opt in adds subscribers immediately when they submit their email address. No confirmation step verifies the address is valid or that the person actually owns it. This is faster but riskier.

Double opt in (also called confirmed opt in) requires subscribers to click a confirmation link in a verification email before being added. This confirms both address validity and ownership.

Double opt in produces higher quality lists with lower complaints and bounces. Single opt in captures more subscribers but includes mistyped addresses, malicious submissions, and potentially unwilling recipients. Most best practices favor double opt in despite lower initial capture rates.