Email error - 550 5.7.1 - Relaying denied. IP name lookup failed

Hi,
when I send an email to a recipient, I get an error 550 5.7.1 - Relaying denied. IP name lookup failed. So far, everything has worked as it should, I do not know where to look for the problem. Can you please advise what to do with it?

The whole error:
--------------------------------

The message did not reach some of the intended recipients.

The following beneficiaries were not reached:

user@example.com

Server Error: 550 5.7.1 ... Relaying denied. IP name lookup failed [8.8.8.8]

REPLY

Hello,

repair is simple - you will have incorrectly set authorization for sending via SMTP server, resp. you probably forgot to set the send authorization in Outlook / Thunderbird.

In Outlook, this is part of the "Server for outgoing connections requires authentication" more here