Same domain only : smtp; 5.1.0 - Unknown address error 530-'SMTP authentication is required

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http://serverfault.com – I have a very strange problem after moving my netwave.be domain from WebHost4Life to Arvixe. I configured several email adresses, like [email protected] and [email protected] For POP3 I can use mail.netwave.be, a mailserver hosted by Arvixe. However, for SMTP I have to use relay.skynet.be. Skynet (Belgacom) is one of the biggest internet providers in Belgium and blocks smtp requests to external mailservers. So for years I've been using relay.skynet.be to send my messages using [email protected] as the sender. The worked perfectly. After moving my domain to Arvixe, this is no longer the ca ()