This guide presents the email authentication mechanisms and the 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 called "envelope sender"), which is used by the mail server to manage 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 a mail software/client, you must authenticate using a username (= your email address) and a password.

If, after authentication, the sender's 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 forgery. It is this inconsistency that triggers the "Sender Mismatch" error.

To correct this error:

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