Cold emailing techniques/habits that have worked for you?

I just started systemizing and scaling up our cold email sales outreach and thought I'd see if anyone here has any tips or tricks that worked for them to improve open rate, CTR, and calls booked.

We're using Snov.io to scrape emails and for the email drip campaigns. Have any of you experimented with cold-calling email leads that click the link in your emails but don't reply? Are there any other B2B services businesses that have found a particular subject line and approach in the email body particularly effective for getting replies?

I'm testing 2 different email flows initially. One more focused on a strong customer success story, and one short and sweet asking for a referral to the right person in the company.

For context - I run a Google, Facebook, and LinkedIn Ad agency focused on helping positive impact, ethical businesses snowball their sales.