While the default mode for most email clients is HTML, plain text format emails are still useful and simplistic.
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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.
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From the left pane, scroll down to Messages and composing.
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From the right pane, go to the Composing section.
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Over here, change the Composer mode from Normal to Plain text from the drop-down. The changes will be auto-saved.
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To check, open a message window for composing emails. The mode should be plain text rather than normal.
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Happy emailing.