[underdown]

Emojis in subject line reliably increase open rate in my experience. Also others report the same:

https://www.outreach.io/blog/will-emojis-increase-reply-rate...

Maybe SO emails go over the top though

[derenge]

How much did they increase your open rate?

The linked report actually indicates a decrease in replies.

  Combined across all seven test variations, we saw a 42% decline in reply rate when the subject line featured emojis.

[underdown]

Tbh I've never optimized for reply rate so this could have been true for me as well. Generally selling dtc. Open rate and sales increased reliably 10-15%. Haven't done any comparable email marketing in two years so things may have changed since then and obviously my anecdata may not be true for you.