Double Opt-In (DOI):
- Subscriber clicks confirmation link after signup
- Eliminates typos, bots, fake addresses
- Produces higher-quality lists
- 20-30% drop-off at confirmation step
- Effectively required in Germany, strongly recommended for GDPR
Single Opt-In (SOI):
- Adds subscribers immediately
- Faster growth, but higher risk of invalid addresses
- CAN-SPAM (US) allows opt-out model with no prior consent
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