Hi @SLau , the general advice I would give here is that: the best volume of comms is stopping when you no longer have anything relevant to say.
For some companies and industries, an email a day might be perfect because it is all relevant content that the recipient values. For others, it might be once a month - since that's how often new content is created.
Unfortunately, there isn't a one-size-fits-all approach, or even much guidance you can give - because it varies so much by company and even individual subscriber. Many companies choose to segment people by engagement, and more engagement = more content sent, since that means they are more likely to value it.
I would defer to the customer journey and the "problem they are trying to solve". Create emails based on that, send them at appropriate times, and you will be fine 🙂
As for measurement, you can see when unsubscribe rates spike, but this will be highly dependent on how much content a person has received, the actual content itself, itself. Without quite a sophisticated testing setup it will be difficult to separate the effect of the number of comms vs the content itself. Best way to do this with simple tech is to split all people into groups of something like "one newsletter monthly", "two newsletters monthly", "one newsletter a week" etc etc, and then measure engagement and unsubscribe rates. But it's tricky!
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