A past customer is not a permanently available sales opportunity. They may have no current need. The useful question is whether there is something timely and relevant you can help them with now.
Group your records by service, completion date and likely next need. A customer whose yard was redesigned may need seasonal maintenance. A commercial client may have another site. Someone who postponed a renovation may be ready to revisit it, if they agreed to future contact. Someone who opted out should remain out.
Choose a real reason
Useful reasons include a service interval, a related project the customer mentioned, an agreed planning date or a genuine opening in your schedule. “We have not emailed you in a while” is not much of a reason from the customer's perspective.
Hi [name], we completed [work] for you in [month]. You mentioned [related need] might be something for later. Is it still on your list? If so, I can explain the options and our current planning window. If not, no problem.
For recurring work, make the next step easier: “Would you like us to check availability for the same service?” Avoid asking them to describe information you already hold, but confirm anything that may have changed.
Run a small first batch
Choose a manageable group you are permitted to contact. Personalize the context, record the date and note the outcome. Some replies will be “not now.” Ask whether they want a reminder at a specific time; do not assume permission for an indefinite sequence.
Measure useful conversations, suitable projects, bookings and contribution after delivery costs. Ten replies saying hello are pleasant but do not establish a profitable campaign.
Important distinction: a message about an existing job and a promotional campaign are different. Follow the applicable rules for your location and channel, and honor opt-outs. Practical privacy and outreach resources appear in the source notes.
Your output: a small customer segment, a reason to contact it and one message. If you cannot explain the relevance in a sentence, choose another segment or wait.