The assumption matters because it determines the approach. If you believe somebody left unhappy, the instinct is to apologise or to offer a discount, and both are wrong when the actual reason is that they simply have not needed you. A message that opens with an apology for a problem that did not occur is confusing and slightly awkward.
Ask a question rather than making an offer. Something short asking whether their circumstances changed, or what they are doing about the thing you used to handle, produces a reply considerably more often than a promotion. It is easy to answer honestly, and somebody who has been meaning to get back in touch is given an easy route.
Segment before writing, because dormant means different things depending on what somebody bought. A customer of a service purchased every few years is not lapsed at eighteen months. One who bought monthly and stopped three months ago is. Applying a single threshold produces messages to people who were never gone and misses people who left quietly.
Prioritise by value rather than by recency. The customers worth a personal message are the ones who spent meaningfully, referred others, or were straightforward to work with, and those deserve an individual note rather than a campaign. For a small business that list is short enough to handle personally, which is considerably more effective than anything automated.
Make it easy to say not now, because that answer is useful. Somebody who tells you their situation changed and to come back in six months has given you a date, which is worth more than a non response. Including that option explicitly increases the reply rate because it lowers what a reply commits them to.
Look for the pattern across everybody who stopped, since that is where the real finding sits. If several stopped at the same point in the relationship, after a particular service, or following a change you made, that is information about the business rather than about individuals. It is also invisible from any single conversation.
Accept that some are genuinely gone and stop pursuing them. Two or three messages establishes whether there is anything there, and continuing past that produces complaints and damages your sending reputation. Somebody who did not respond has answered.
Then act on what you learn rather than only on who returns. The most valuable outcome of this exercise is frequently not the recovered customer but the sentence somebody writes explaining why they drifted, and that sentence usually names something you can fix for everybody currently buying.
Time the approach against something plausible rather than sending it whenever you notice. The anniversary of their last purchase, the start of a season relevant to what you sell, or a change in your offering all give the message a reason to exist beyond your wish to hear from them.
Record what each one says, because those replies are the most honest description of your business you will receive. Customers who left have no reason to be diplomatic and no ongoing relationship to protect, and a handful of those answers frequently identifies something that everybody currently buying has noticed and not mentioned.
Do it by phone for anybody who mattered, because the reply rate and the honesty are both considerably higher than any message achieves. Five calls to former customers who spent well is an afternoon, and it produces information a survey never will.