My top tip for this would be to put less ‘stuff’ into your emails but send more frequently. People still get hung up on sending a monthly newsletter. Bin that. Focus instead on sending an email when you have something to say or promote, don’t pile your content up and send ten stories in one go, send ten emails with one story in each.
I covered this in our Mailchimp Meetup from 2019. Here are two slides that make this point.
Don’t do this.
Instead, do this.