Test the things that move the needle. Skip the cosmetic stuff. Here's the rough priority order by impact on reply rates and pipeline.
Single biggest driver of open rates. Can swing opens by 30-60%. Test one new subject line per week against the control.
The make-or-break for reply rate. A truly personal first line vs a generic one can 2-3x replies.
The specific angle you're pitching. Same product, different framing can double positive reply rate. E.g., "help your AEs ramp faster" vs "improve forecast accuracy", different buyers care about different problems.
Who the email is "from." Founder vs SDR vs fictional-persona email can shift reply rates. Test this cautiously, sender reputation builds slowly.
Very short (40 words) vs standard (80-100) vs longer (150+). Typical B2B sweet spot is 70-110 words.
Specific times ("Tue 2pm or Thu 10am") vs open ("What works?") vs content offer ("want the summary?"). Different CTAs serve different prospect states.
Tuesday-Thursday mornings work best for most B2B. Monday and Friday underperform. Test within the window.
Recipient-local 8-11am vs 1-3pm. Both work; specific audience may prefer one.
5-touch vs 7-touch. See sequence length.
Minimal vs expanded. Usually doesn't matter much, but heavily marketing-ized signatures hurt.
30/day vs 50/day. Deliverability-related, not copy-related.
"Hey [name]" vs "Hi [name]" vs "[name],". Rarely meaningful differences.
Plain text vs minimal HTML. Plain text wins for cold, but the margin is small.
These changes at typical cold email volumes produce differences smaller than the noise in the data. Optimization theater.
Maintain a running list of hypotheses. Each week or two, test the highest-leverage one. Keep a log:
Over 6 months you build a proprietary testing archive worth more than any playbook. The knowledge is specific to your audience, your offer, your voice.
For small-volume campaigns (under 500 sends), you can't A/B test cleanly. Instead, use qualitative iteration:
Faster than formal testing. Good for early-stage campaigns where you're still figuring out the pitch.