A/B testing is trying two different techniques that communicate the same message on a small percentage of your list, seeing whether A or B performed the best, then using the better performing option when you email the rest of the list. It sounds complicated, but there are plenty of email marketing solutions out there that take all the math and guesswork out of it. (All you have to do is come up with the option A and option B you want to try.)
An email marketing strategy is part of your overall marketing strategy and business plan. It helps you market your products and services with the use of the email channel with the best chances for making a profit and reaching your goals. That is because an effective email marketing strategy takes into consideration what your target customers are, their preferences and benefits they are looking for as well as your products services and industry and which email marketing messages are most effective.
Rob, you don’t say who ‘booted’ you from using it. A significant GDPR factor is non-profits having to consent/re-consent those on established email lists and experiencing significant proportions of lists being lost because people miss the notifications or are too busy to fill in yet more forms. However, I have found a few using a ‘one-touch’ re-subscription button that takes immediate effect, without the recipient having to do anything else. It would appear that the re-subscription rate is higher, the easier it is to activate. On enquiry, I was told that they were using mail chimp for this.
Particular groups of customers can be targeted or even individuals. Offering individual customers special deals on merchandise and/or services on the customer's birthday, for instance, is one example of email marketing personalization. (A restaurant might send an email to customers on their birthday offering 50% off an entree,) Email marketing helps a business develop and maintain a relationship with a customer over time that hopefully results in increased sales and increased customer loyalty. 
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