Deliverability performance is not determined by the message type: an HTML coded email versus a text-based email.
Deliverability performance is determined by numerous variables surrounding your sending reputation and your ability and willingness to follow email marketing best practices. Senders that follow best practices and have highly engaged subscribers are more likely to reach the inbox whether the email is HTML or text-based.
Although HTML or text-based emails are not factors that determine inbox placement, having a balance between the two in your email program may result in a better subscriber experience. Some subscribers prefer or may have their email clients set up by default to receive text-based emails.
If you are concerned that some of your subscribers have email clients set up to accept text-based emails, use the MIME multipart content-type, which allows you to send both an HTML coded email and text-based email within the same message. This makes it possible for subscribers set up to receive text-based messages to see your email. Talk to your email administrator or Email Service Provider to help you set it up.
