A text mail subscriber is a person who specifically requests to receive text emails. As noted, the emails that go out through email clients and ESPs are MIME or multi-part format which provides support for varying content types and multi-part messages as well as determine which format to display for any given recipient. Some devices, like the Apple watch, can only display text emails. But, there are those who wish to only receive text emails, not multi-part and there is a growing trend toward sending simple text email broadcasts to subscribers. Most ESPs offer an option of create a different text version. So, if you’re creating a multi-part email with a big honking video in the middle of it, you will want to create the alternative for the text-only folks.
When an ESP (Email Solution Provider) is used to send text emails, opens and clicks can be tracked.