Email and SMS are not interchangeable. They carry different weight, command different attention, and earn different responses. Treating them as the same channel with a different character count is how lists get burned.
Email is for content and consideration. It carries detail — product education, lifecycle context, a reason to click through and browse. It earns its place by being useful and not arriving more than the customer expects.
SMS is for timing and urgency. It works when there is something the customer needs to act on now: an order is ready, a window for redemption is closing, a behavior-triggered offer expires in hours. Because SMS demands immediate attention, the bar for sending should be higher.
The division of labor is straightforward: email builds the relationship over weeks, SMS accelerates decisions in moments. Use email to teach customers what to expect from you, and SMS to remind them when a moment matters. The mistake is using SMS to repeat what email already said — that is how opt-outs climb.