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Fix an error "relay not allowed
This guide explains how to resolve the error “The recipient abc@domaine.xyz has been rejected by the server, as relaying is not allowed” which may occur if your email address is not configured correctly in your email software/client.
How to fix the error?…
… From an Apple product (Mac, iPhone, iPad, etc.)
The easiest way is to delete the email account in question and use the configuration tool to reinstall it.
Refer to the official help to delete the email account on your Mac.
Reinstall your email with an Apple synchronization profile.
Warning, before deleting the email account, make sure it is configured as IMAP and not POP. Refer to this other guide to switch the account from POP to IMAP.
… From any other email client
Refer to this other guide by following the instructions corresponding to the closest version to the one you have.