I receive emails in plaintext from dev mailing lists (say LKML), so I want to see the emails in a monospace font because otherwise it is useless to see the email since I can't see code changes properly, leading whitespace is removed, and tabs are translated to spaces which makes Zoho Mail client to show corruption of intended email.

You should not touch plain text emails. Also it would be helpful if you could allow to remove indentation in reply to plaintext email (it's plaintext, > already indicates quote, and thus space on its left is superfluous, and it is not shown when seeing the email), and allow bottom-posting instead of top, which is an 8 years old problem: https://help.zoho.com/portal/en/community/topic/reply-to-email-top.