Thanks for your feedback. Unfortunately, it is unlikely that we will be offering a single opt-in option. Here's why....
A greater risk to our sender reputation
If we were not to use a double opt-in system, there's a risk that a number of bad addresses can enter into your subscriber list. This includes mistyped email addresses, which can impact our sender reputation by adding to the statistics of undeliverable addresses that we're attempting to mail to.
We also risk having somebody who is not the owner of an email address entering the address into your subscription box as a joke. When the actual owner of that email address receives the email, he or she may mark the email as spam which also would negatively impact our sender reputation.
We believe that a double opt-in ensures a quality subscriber list.
Finally, effective 25 May 2018, the European Union's General Data Protection Regulation (GDPR) takes effect for anyone having subscribers located in the EU. Double opt-in provides additional proof of consent, which protects any publisher with a subscriber who happens to be located in the European Union.
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If you would like to use Mailchimp, feel free to take a look at this help article.