The original message was received at Wed, 20 Dec 2017 09:53:09 +0100 from lx21.zim.uni-duisburg-essen.de [132.252.180.18] ----- The following addresses had permanent fatal errors ----- <david.immel(a)stud.uni.due.de&gt; (reason: 550 5.2.0 No such mailbox) ----- Transcript of session follows ----- ... while talking to smtpin.rzone.de.: <<< 550 5.2.0 No such mailbox 550 5.1.1 <david.immel(a)stud.uni.due.de&gt;... User unknown <<< 554 5.5.0 No valid recipients have been specified