Plain text only. No images, no HTML formatting, no attachements. This makes it look like a personal email from a real person.
No open tracking pixels: If you want to track open rates, then the pixel added in it might cause the emails to go to spam. Moreover, certain email service providers download this pixel by default, thus giving a wrong open signal. It is better to optimize for replies and meetings that track open rates.
No links in the first email. Links trigger spam filters. Avoid adding links in the first email.
Second email onwards: one link maximum. From email 2, you can include one link — a calendar link, case study, or website URL. More than one link increases spam score.
Short subject lines. 3–5 words. Avoid sales language ("Increase your revenue", "Quick question"). Treat the subject line like the first line of the email.
Short emails. 3–5 sentences maximum for a first email. Problem-Solution-Offering often work well.
Clear CTA: Have one clear ask from the customer. Asking an open-ended borad question or asking multiple questions, get fewer replies.