docs(email): enhance clarity on replaceToRecipient behavior

Updated the documentation for `replaceToRecipient` to provide clearer instructions on its usage and implications, particularly regarding empty `to` fields with cc/bcc for Custom SMTP. Added structured formatting for better readability.

Author: jainpawan21

content/docs/platform/integrations/email/index.mdx

@@ -202,7 +202,21 @@ await novu.trigger({
 </Tab>
 </Tabs>
 
-<Callout type="info">By default, Novu merges override `to` addresses with the subscriber email. Set `replaceToRecipient: true` to send only to the override recipients and omit the subscriber email from `to`. When `replaceToRecipient` is `true`, include at least one of `to`, `cc`, or `bcc` in the email override. If `to` is explicitly set to `[]` and `cc` or `bcc` is set, Novu keeps `to` empty and delivers via `cc`/`bcc` only. For Custom SMTP (Nodemailer) providers, Novu normalizes an empty `to` to `undisclosed-recipients:;` so the message sends without exposing subscriber or sender addresses in the `To` field.</Callout>
+<Callout type="info">
+
+**`replaceToRecipient`**
+
+- By default, Novu merges override `to` addresses with the subscriber email.
+- Set `replaceToRecipient: true` to send only to override recipients. The subscriber email is omitted from `to`. `to` is replaced with the `from` address.
+- When `replaceToRecipient` is `true`, include at least one of `to`, `cc`, or `bcc`.
+
+**Empty `to` with cc/bcc (Custom SMTP only)**
+
+- Setting `to: []` with `cc` or `bcc` is supported only on **Custom SMTP (Nodemailer)**.
+- Novu normalizes an empty `to` to `undisclosed-recipients:;` so the message sends without exposing addresses in the To field.
+- Other email providers do not support this pattern.
+
+</Callout>
 
 ### Targeting a specific provider