Treat your email subscribers like gold. Only ever send them high quality, high value email communications.
Sending an email to a list of “opt-in” subscribers is a relative cheap way to market your business. Often, getting the email ready takes more work than you planned and you get to the point of “we just need to get this sent out”. If that’s the case, take a mini break and then have a fresh look to make sure you’re preserving the quality of your email communications.