Ik krijg regelmatig onderstaande melding als ik mail stuur of beantwoord.
Wat gaat er fout?

This is the mail system at host ewsoutbound.kpnmail.nl.

I'm sorry to have to inform you that your message could not be delivered to one or more recipients. It's attached below.

<……………...> failed: host  d307845.a.ess.de.barracudanetworks.com (18.185.115.252) said: 550 permanent  failure for one or more recipients (………………………..:blocked) (in  reply to EOD command)