I wouldn't recommend email marketing in this instance for 2 reasons:
1. If you buy a list and send the email as "bulk" i.e. the same email goes to more than one recipient, then you are actually spamming, as they haven't opted in to receive your emails. The definition of spam is "bulk" i.e. more than one, and "unsolicited" - they haven't asked to receive your emails.
However, it is legal to send individual, unsolicited emails. If a list supplier tries to tell you that the email addresses are "opted in", then, by my understanding, they're not telling you the whole truth. They've probably opted in to "something" - but not specifically you. AVOID.
2. Unsolicited email has incredibly low response rates. One company spent £1600 to send 6,000 emails and got one response which didn't convert into a sale. Wow - that's someone's payrise gone on an interesting experiment in email marketing, which could have been avoided with proper advice!
If I understand correctly, your goal is to get more regular contracts of reasonble value, and you don't need 100s or 1000s of responses. Therefore, I would suggest direct mail to a very highly targeted list (which you seem to have already) and to spend more money and time per recipient, than a mass, unfocused random emailing or mailing.
A way to approach it is this:
1. Don't try to sell the contract or your services on the first contact. I would offer some compelling reason for them to get in touch with you. You can either offer a free report e.g. solving a common landscaping/maintenance problem or how to avoid being ripped off by ... etc. OR some kind of free audit or diagnostic. The aim at this stage is to begin the relationship and to start building a list to follow up with.
2. Don't expect high response unless your offer is insanely juicy. You are better to mail 10 people 3 times, than 30 people 1 time. Pick your highest probability prospects and write 3 letters emphasising different aspects of your offer each time, or even have 3 different offers.
3. Create a follow up sequence for those that respond. Build the relationships by staying in touch - educate your prospects and demonstrate your expertise and professionalism. Become the "go to guys" for grounds maintenance. If you have crafted your offer in such a way that your prospect gives you his/her email address, then you can follow up more cheaply by email.
You might also consider following up by phone, but I think this tends to work better once you've sent them some requested information - you can call to check they received and ask them if it was useful and do they have any questions etc. Just be wary of not being seen as a "telemarketing pest" - you need to be perceived as adding value.
4. Make offers in your follow up and you will make sales.
5. Test and tweak
A cautionary tale: One client sent 2000 letters and only got one response (which did actually convert into business). That is a pitiful response rate. What could she have done differently?
1. Changed the offer from selling the whole enchillada of her service to a smaller, initial step
2. Copywritten the letter better
3. Written to fewer people more regularly - people aren't always ready to buy right now
HTH,
Jane