Hello, can you provide an eml ?

Not at the moment, but I think that may not be necessary. I've found a way by accident to show what's being ignored by Thunderbird and other mail clients, but not by SOGo: I've taken the base64 encoded body, ran it through base64 -d and looked at the result through emacs. I've previously had base64 put the output to stdout which doesn't show these extra symbols. At the line breaking point where SOGo breaks the line, emacs shows the character combination "^M" which probably is some kind of new line character. SOGo puts a line break everywhere this character appears, while Thunderbird ignores it. Now I'm not sure about the specifics - or even why Thunderbird but them there in the first place - but for viewing the content, at least Thunderbird and K9 mail only insert an empty line when there's only the "^M" in a line.