Despite the best intentions of the internet's innovators spammers and their relentless delivery of mindless rubbish have managed to make most people extremely wary of sharing a clean email address.
Information of value that help to build trust
If you are lucky enough then as a business to have a list of people willing to receive your messages they need to be treated with respect. You know them, but not well enough to be too familiar and this must form the basis for your communication until a level of trust has been established.
Say something interesting and say it quickly
Messages at this point in the relationship need to be short, pleasant and have only one objective - building trust. Impart information that is of genuine use. Encourage engagement through honesty. Introduce a benefit that you could genuinely bring were they a client, a problem you might be able to help with or an idea that might inspire action on their part.
And remember that an opened email without a subsequent unsubscribe click might not seem much but takes your business a small step closer to building the relationship and growing trust and with drip campaigns that's exactly what you are trying to achieve.