You will not be able to track emails in plain text mode as it strips all images (and formatting) from the email which is used by Ebsta for tracking.
Switch to Rich text or HTML formatting in Gmail
To switch from plain text to rich text formatting in Gmail, open the compose window and click the more actions button in the bottom right-hand corner and uncheck Plain text mode.
Switch to Rich text or HTML formatting in Outlook
If you are using the desktop version of Outlook:
1. Click File > Options.
2. Select Mail.
3. In Compose messages, click the drop-down arrow next to Compose messages in this format and choose the format you want to use as the default for new emails.
If you are using Outlook online:
1. Click Settings
2. Select Email > Compose and reply.
3. In Message format, click the drop-down arrow next to Compose messages in...format and choose the format you want to use as the default for new emails.