I'm getting a specific error (RULE3_2,...) when trying to send emails generaly or a particular one.

Our anti-spam system assigns a score to every message submitted to our SMTP service (mail.gandi.net or via our webmails). If this score exceeds a certain threshold, your message may be blocked. This is designed to ensure the stability of our SMTP service. If you are unable to send an email you consider legitimate, please refer to our documentation. If the problem persists, open a support ticket at the bottom of this page and provide us with the details of the message in question ( email address of the sender, recipient, specific date and time the attempt was made, etc.).
There are many reasons why a message may be blocked: the subject of the message, the content of the message, any attachments, the IP address from which the message was sent, or a combination of these factors.

Outgoing email errors