While the default mode for most email clients is HTML, plain text format emails are still useful and simplistic.
Here is how to enable plain text mode in Proton Mail for sending messages:
Login to Proton Mail and click the gear icon from the top right. Then, select All settings.
From the left pane, scroll down to Messages and composing.
From the right pane, go to the Composing section.
Over here, change the Composer mode from Normal to Plain text from the drop-down. The changes will be auto-saved.
To check, open a message window for composing emails. The mode should be plain text rather than normal.
Happy emailing.