2 recommended

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Mailchimp A/B

Pius BinderChannel Marketer, Facebook · Written
Hi @Matej I think A/B testing mailchimp is a good solution but not 100% sure whether this feature is available for free version: https://mailchimp.com/features/a...
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Mailchimp by far. They have a low cost of entry (it will grow as your non-profit grows) and they have built in a/b testing capabilities. They may even offer discounts for non-profits, just ask.
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Optimizely

AleksandraProduct Marketing · Written
Optimizely for sure is a very powerful platform for experiments. They now have a bunch of products/solutions for different needs and problems. Though they are not free, but well worth the price.
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