Worked for me by adding:
MIME-Version: 1.0
Content-type: text/html; charset=UTF-8
at the END of subject. putting the above code at the beginning of subject leads to have subject with "MIME-Version 1.0" in the email.
Besides it didn't work via tool > email > send-email Winbox GUI.
I also escaped quotation marks in the body. (\")
Example:
/tool e-mail/ send to="email@example.com" subject="Welcome to Stripe\r\nMIME-Version: 1.0\r\nContent-type: text/html; charset=UTF-8" body="<!DOCTYPE html PUBLIC \"-//W3C//DTD XHTML 1.0 Transitional//EN\" \"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd\"><html dir=\"ltr\" lang=\"en\"><head><meta content=\"text/html; charset=UTF-8\" http-equiv=\"Content-Type\"/><meta name=\"x-apple-disable-message-reformatting\"/></head><div style=\"display:none;overflow:hidden;line-height:1px;opacity:0;max-height:0;max-width:0\">You're now ready to make live transactions with Stripe!<div> </div></div><body style=\"background-color:#f6f9fc;font-family:-apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,"Helvetica Neue",Ubuntu,sans-serif\"><table align=\"center\" width=\"100%\" border=\"0\" cellPadding=\"0\" cellSpacing=\"0\" role=\"presentation\" style=\"max-width:37.5em;background-color:#ffffff;margin:0 auto;padding:20px 0 48px;margin-bottom:64px\"><tbody><tr style=\"width:100%\"><td><table align=\"center\" width=\"100%\" border=\"0\" cellPadding=\"0\" cellSpacing=\"0\" role=\"presentation\" style=\"padding:0 48px\"><tbody><tr><td><img alt=\"Stripe\" height=\"21\" src=\"https://logos-world.net/wp-content/uploads/2021/03/Stripe-Logo-700x394.png\" style=\"display:block;outline:none;border:none;text-decoration:none\" width=\"49\"/><hr style=\"width:100%;border:none;border-top:1px solid #eaeaea;border-color:#e6ebf1;margin:20px 0\"/><p style=\"font-size:16px;line-height:24px;margin:16px 0;color:#525f7f;text-align:left\">Thanks for submitting your account information. You're now ready to make live transactions with Stripe!</p><p style=\"font-size:16px;line-height:24px;margin:16px 0;color:#525f7f;text-align:left\">You can view your payments and a variety of other information about your account right from your dashboard.</p><a href=\"https://dashboard.stripe.com/login\" style=\"line-height:100%;text-decoration:none;display:block;max-width:100%;background-color:#656ee8;border-radius:5px;color:#fff;font-size:16px;font-weight:bold;text-align:center;width:100%;padding:10px 10px 10px 10px\" target=\"_blank\"><span><!--[if mso]><i style=\"mso-font-width:500%;mso-text-raise:15\" hidden> </i><![endif]--></span><span style=\"max-width:100%;display:inline-block;line-height:120%;mso-padding-alt:0px;mso-text-raise:7.5px\">View your Stripe Dashboard</span><span><!--[if mso]><i style=\"mso-font-width:500%\" hidden> ​</i><![endif]--></span></a><hr style=\"width:100%;border:none;border-top:1px solid #eaeaea;border-color:#e6ebf1;margin:20px 0\"/><p style=\"font-size:16px;line-height:24px;margin:16px 0;color:#525f7f;text-align:left\">If you haven't finished your integration, you might find our<!-- --> <a href=\"https://stripe.com/docs\" style=\"color:#556cd6;text-decoration:none\" target=\"_blank\">docs</a> <!-- -->handy.</p><p style=\"font-size:16px;line-height:24px;margin:16px 0;color:#525f7f;text-align:left\">Once you're ready to start accepting payments, you'll just need to use your live<!-- --> <a href=\"https://dashboard.stripe.com/login?redirect=%2Fapikeys\" style=\"color:#556cd6;text-decoration:none\" target=\"_blank\">API keys</a> <!-- -->instead of your test API keys. Your account can simultaneously be used for both test and live requests, so you can continue testing while accepting live payments. Check out our<!-- --> <a href=\"https://stripe.com/docs/dashboard\" style=\"color:#556cd6;text-decoration:none\" target=\"_blank\">tutorial about account basics</a>.</p><p style=\"font-size:16px;line-height:24px;margin:16px 0;color:#525f7f;text-align:left\">Finally, we've put together a<!-- --> <a href=\"https://stripe.com/docs/checklist/website\" style=\"color:#556cd6;text-decoration:none\" target=\"_blank\">quick checklist</a> <!-- -->to ensure your website conforms to card network standards.</p><p style=\"font-size:16px;line-height:24px;margin:16px 0;color:#525f7f;text-align:left\">We'll be here to help you with any step along the way. You can find answers to most questions and get in touch with us on our<!-- --> <a href=\"https://support.stripe.com/\" style=\"color:#556cd6;text-decoration:none\" target=\"_blank\">support site</a>.</p><p style=\"font-size:16px;line-height:24px;margin:16px 0;color:#525f7f;text-align:left\">— The Stripe team</p><hr style=\"width:100%;border:none;border-top:1px solid #eaeaea;border-color:#e6ebf1;margin:20px 0\"/><p style=\"font-size:12px;line-height:16px;margin:16px 0;color:#8898aa\">Stripe, 354 Oyster Point Blvd, South San Francisco, CA 94080</p></td></tr></tbody></table></td></tr></tbody></table></body></html>"
Result:
sc1.png
sc2.png
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