I tend to use a mix of in person conversation, phone call, and email for referrals. If I already have an established relationship with someone where I know enough about the business service(s) that they offer, I'll do an introductory email with the referral and then follow up with a call to the member if I haven't seen an email acknowledgement in a reasonable timeframe. If I have an idea of the business services that someone offers, but I'm not sure whether the needs of the person I want to refer fit, I typically ask clarifying questions about the business services in person, by phone call, or by email before I'll pass along the referral. The goal is ensure that mutual benefit is more likely.