A little time ago, I had a conversation with one of the Disroot maintainers through the fedi in which that person suggested it was discussed the possibility to have a customized domain for Delta Chat users.
Sorry, I don’t understand what you mean here. I am just notifying Delta Chat users that Disroot is making some good changes for them. Also, I don’t know about posteo.de nor have any contact with them even nor to know if they are using free software on the server side like Disroot.
Fede from Disroot is maintaining contact with some Delta Chat users from a XMPP room using a bridge and the intentions were already clarified. The domain is going to be provided nearly for sure and meanwhile Disroot are whitelisting accounts for Delta Chat use under request.
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I mean, if Posteo is doing the same, good for them but the way in which you propose that about what I said is not the best way to start a conversation.