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A good practice is to create a triggered email ( Marketing+ > Triggered messages) notifying those subscribers that you'll be reducing mailing frequency. The Event trigger is setup with the condition to send the email if 90 days pass since their last click or open (as seen below). The copy of the triggered email can be something in the lines of "We've seen that lately you've shown little interest in our emails. For now, we're going to send you fewer emails and try to make or our emails more relevant to you!"
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