This guide presents the email authentication mechanisms and ways to resolve the Sender Mismatch error.

Introduction

• The "Sender Mismatch" error occurs when the address used to send an email does not match the authentication address used when connecting to the SMTP server.
• This error is related to two specific elements:
  • the Return-path (also called "envelope sender"), which is used by the mail server to manage returns and delivery errors,
  • and the From header, which is the sender address visible to the recipient in the message.

Troubleshooting the Sender Mismatch Error

Technically, when sending an email via a software/email client, you must authenticate using a username (your email address) and a password.

If, after authentication, the sender address specified in the "From" or "Return-path" header is different from the address used for authentication, the Infomaniak server may consider this a forgery attempt.This inconsistency triggers the "Sender Mismatch" error.

To fix this error:

• Make sure that the email address used for sending is the same as the one you use to log in (or a valid alias).
• Do not modify the Return-path address or the From header to use another unauthorized address (if necessary, please refer to this other guide to test another email application or configure your current email software/client correctly).