fix(tem): email content guidelines (#5092)

Author: ldecarvalho-doc

pages/transactional-email/reference-content/understanding-tem-reputation-score.mdx

@@ -68,11 +68,12 @@ For a good domain reputation, you need to implement the following emailing pract
 - Use the [preheader](/transactional-email/concepts/#preheader) to entice your recipients to open your email.
 - Do not use spam phrases in the **subject line** of your emails. Spam words and phrases associated with scams, promises and free gifts arouse suspicion among email service providers.
 - Maintain a 60/40 text-to-image ratio.
-- Send emails with a coherent structure.
 - Keep a clean list of **engaged users** and delete email addresses of recipients that do not engage with your content.
 - Do not purchase an email list as they might include [spam traps](/transactional-email/concepts/#spam-trap).
 - Delete emails that have [soft bounced](/transactional-email/concepts/#soft-bounce) and [hard bounced](/transactional-email/concepts/#hard-bounce).
 - Use **shared IPs** if you are sending small volumes of emails (< 5 000 emails/day). A shared IP allows you to send emails without needing to warm up your IP, and improve your deliverability by pooling all its users' IP reputation. This also ensures that your reputation does not suffer if you are not sending enough emails consistently from your IP.
+- The sending domain should be the same as the public website.
+- Follow the [email content guidelines](#email-content-guidelines) listed below.
 
     <Message type="important">
     - If you are sending large volumes of emails (> 300 000 emails/month), consider using a dedicated IP (not yet provided by Scaleway). This ensures that the emails you send are not affected by the reputation of senders using the same IP address.
@@ -81,4 +82,40 @@ For a good domain reputation, you need to implement the following emailing pract
 
 <Message type="tip">
  Find out [how to use your **Email activity** tab](/transactional-email/how-to/manage-email-activity/#how-to-use-your-domains-email-activity-tab) to manage your domain reputation.
-</Message>
+</Message>
+
+### Email content guidelines
+
+To maximize deliverability, we recommend your email content respects the following guidelines:
+
+- Keep the email content short and concise.
+- The email should be either in plain text (text/plain) or in both plain text and HTML (text/html). If both versions are present, the text should be identical in both.
+
+**Links**
+- Avoid including too many links.
+- Links should not point to external domains. External domains are any that do not point to the sending domain. If you need to link to an external domain, use a redirection.
+- Subdomains of the sending domain are allowed.
+- All links should use a secure connection (HTTPS).
+- Do not put links under hidden content.
+
+**CSS**
+- CSS styles should be included directly in the HTML code to ensure compatibility with most email clients.
+- Keep CSS simple and organizational.
+- You should provide the option to view the email in a web browser.
+- Avoid hiding content using CSS (e.g., display: none).
+
+**Personal Information**
+- Avoid including personal information, such as names or addresses, in the subject line.
+- Keep personal information in the email content to a minimum.
+- Avoid including phone numbers as they may be blocked.
+
+**Vocabulary**
+- Avoid using financial or monetary vocabulary (e.g., money, finance, credit, free, guarantee).
+- Including vocabulary of a sexual nature and connotation will greatly impact your reputation and deliverability. Avoid using it at all costs.
+
+**Images**
+- Avoid using images as much as possible.
+- Ensure the text-to-image ratio is greater than 40% if you use them.
+- Add an "alt" (alternative text) tag to images.
+- Keep images light and compressed.
+- Avoid using hidden pixels.