Are there any ISPs or email clients that only accept text emails and reject HTML emails?

Email marketer from Campaign Monitor shares that while not rejecting HTML outright, some spam filters penalize emails that don't have a plain text version. They suggest using a multi-part MIME format.

Email marketer from Email on Acid explains that some email clients allow users to choose whether to view HTML emails, so emails should have a text version.

Email marketer from Litmus shares tips for properly coding HTML emails, emphasizing the importance of inline CSS, using tables for layout, and providing a fallback plain text version. They don't mention any email clients only accepting text.

Email marketer from Reddit explains that while no major ISPs strictly reject HTML emails, excessively large or poorly coded HTML emails can trigger spam filters. Sending both text and HTML versions (multipart/alternative) is recommended for better deliverability.

Email marketer from Stackoverflow explains that while it's technically possible for a server to be configured to reject HTML emails, it's highly unusual. Most servers will either deliver both HTML and text versions or convert HTML to text.

Email marketer from Quora shares that some people still use text-only email clients like 'mutt' because they are more secure, lightweight, and customisable. They can read HTML emails but choose not to.

Email marketer from Super User shares that command line email clients and some older email clients may only support plain text emails or require configuration to view HTML.

Email marketer from Email Marketing Tips blog explains that while no major ISPs block HTML emails entirely, having a plain text version increases deliverability due to spam filters favouring multi-part MIME emails.

Email marketer from Mailjet explains that having a plain-text version alongside HTML in emails is crucial for deliverability as it is friendlier to spam filters, not because ISPs block HTML.

Expert from Word to the Wise, Laura Atkins, discusses the importance of sending both HTML and plain text versions of emails. While she doesn't state that ISPs outright reject HTML emails, she emphasizes that having a plain text alternative can improve deliverability as it caters to users who prefer or require plain text and can help avoid spam filters.

Expert from Email Geeks states she has never heard of SMTP blocking like that, but some clients only display the plain text part of the email due to user choice, not blocking.

Expert from Email Geeks shares his opinion that in 2021, it is not worth doing multipart/alternative in emails. He suggests just sending text/html, because the small amount of recipients who can't read text/html isn't worth the extra complexity. However, he says plain text emails are fine, and it's only multipart/alternative emails he would be OK with getting rid of.

Documentation from Apple outlines how users can view all messages in plain text format within the Apple Mail application. It does not block HTML emails, it is a user preference setting.

Documentation from Mozilla clarifies how Thunderbird users can configure the email client to display messages in plain text rather than HTML for security or preference reasons.

Documentation from RFC 2046 outlines the standards for Multipurpose Internet Mail Extensions (MIME), including 'text/plain' and 'text/html' content types, which indicates that both plain text and HTML emails are widely supported.

Documentation from IETF RFC 5322 defines the Internet Message Format. It specifies the 'Content-Type' header field, used to indicate the format of the message body (e.g., 'text/plain' or 'text/html'). It doesn't specify rejection of different content types.

Documentation from Microsoft Support details how Outlook can be configured to read all email messages in plain text. This is a user-level setting and not a server-level rejection of HTML emails.