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Is it good practice to always have a plain text equivalent of the HTML email?
What is worst case scenario only having the HTML email?
Sendy sends 'multi-part' email, that means your email contains plain text and HTML versions. If the recipient's email client doesn't support HTML emails (pretty rare), it'll display the plain text version. If you don't populate the plain text version of your email, your email will be blank on email client's that only shows the plain text version.
If you don't wish to populate the plain text version, at least include a line of text in the plain text field that says:
Click here to view the web version of this email - [webversion]
So at least they can view your email on the web.
Hope this helps.