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

Preamble

• 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 known as "envelope sender"), which is used by the mail server to handle returns and delivery errors,
  • and the From header, which is the sender's address visible to the recipient in the message.

Resolve the Sender Mismatch error

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

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

To fix this error:

• Check that the email address used for sending is the one you authenticate with (or a valid alias).
• Do not change the Return-path address or the From header to an unauthorized address (if necessary, refer to this other guide to test another email application or to correctly configure your current email software/client).