To properly send HTML mail with a plain text alternative from October CMS, follow the steps given below:
Open your October CMS backend
Go to Settings > Mail > Mail templates.
Create a new email template with both HTML and plain text content sections.
Use the ‘Mail::send()’ method to send the email, specifying both HTML and plain text views:
Create two views: one for HTML ‘template_html.htm’ and one for plain text ‘template_text.txt’.
Store them in the appropriate plugin folder
/plugins/author/pluginname/views/mail
Ensure the email is sent with both HTML and plain text alternatives by sending it to yourself and checking different email clients.