2. Forgetting About Mobile
Between 2013 and 2014, mobile visits to Artsy.net increased by over 350%. Additionally, over 60% of U.S. consumers now read emails on a mobile device. Since your response is more likely to be viewed on a smartphone than a PC, make sure your gallery emails are mobile-optimized. If you follow the advice above (clean visuals, succinct message, in-line images), your emails should already be mobile-friendly.
Tip: To ensure your emails look great on mobile, send a test response to yourself and a few colleagues. Note the differences on different devices.