Address in mailbox given [hostmaster@] does not comply with RFC 2822, 3.6.2.)

Server
2

Hey,

i did the following search:

https://central.owncloud.org/search?q=%22Address%20in%20mailbox%20given%22%20%22does%20not%20comply%20with%20RFC%22%20order%3Alatest

and already found quite some existing threads discussing the same message where the first one after yours seems to already include a possible fix: