The reason my cold email had a 40% response rate is that I sent only nine such emails.

In the past, I sent out fifty emails and received a reply from only two individuals. However, last month I sent nine emails, all of which referred to an aspect in relation to their product or a recent blog that they had posted, and received four actual replies. The problem wasn't about volume; it was relevance.

For those conducting outreach manually, what determines the point at which your list is worth the effort of additional research per email versus just sending out more?