Yes, it is possible, although it has to be in HTML, so it’s not completely the same as plain text. To use your own HTML you have to mark this checkbox:
Unfortunately, due to the structural email regulations, Sender.net can not allow fully plain-text emails to be sent.
Every email must have an unsubscribe link, otherwise it will be placed to SPAM or worse – won’t be delivered at all. Because of that – every email has to be converted to HTML.
If your subscribers are using a provider that does not support HTML, they’ll receive a plain-text version of the email, which you can check and edit in the last step of the campaign creation: