What actually improves email open rates?

What are people doing to make email marketing work when batch sending still struggles?

We recently switched domain names in an effort to improve our email open rates, as most messages sent from our previous domain appeared to be not reaching the inbox due to spam filtering.

This week, we ran a campaign to a very targeted list promoting a webinar with a well-known industry influencer. Everyone on the list knows him.
Subject line (name changed):

“Coffee with Joe Schmo – Turn equipment into a top profit driver in 2026!”

To be cautious, we sent the emails in small batches—around 30 emails at a time, spaced 15 minutes apart, for a total of 10 batches.

Despite that, our open rate was only 12%, and we haven’t seen any noticeable improvement since switching domains.

I’d really appreciate hearing what’s worked (or hasn’t) for others. Thanks in advance :-)