The debate around HTML vs. plain text email still continues especially with the rise in mobile device use. Plain text email however, is probably the most prevalent in transactional email where companies send very personalized, 1:1 messages in response to key actions that their customers take, such as order confirmations, account notifications and friend requests. Nowadays, many companies are expanded their transactional email to include more marketing based content since transactional email tends to have the highest open rates. But at the end of the day, the battle between HTML and plain text is really one (or won) between imagery and words.
To help in your evaluation, it would help to first explore the pros and the cons of plain text email. Here are the pros.
But there are a few drawbacks.
When deciding whether or not to use plain text emails, think about your goals. Overall, you should typically use plain text when dealing with b2b mail. Corporate email delivery can be tricky due to custom spam filters and firewalls. Additionally, mobile devices favor plain text email, especially smartphones and blackberry devices. In the end, you should test them both to see which one gains you maximum response. You just might be surprised at the results.