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Resolve a "relay not allowed" error
This guide explains how to resolve the error “The recipient abc@domaine.xyz has been rejected by the server, as the relay is not authorized” which may occur if your email address is not configured correctly in your software/email client.
How to resolve the error?…
… From an Apple product (Mac, iPhone, iPad, etc.)
The simplest solution 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.
Attention, before deleting the email account, make sure it is configured in IMAP and not in 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.