having this problem since a while, so it’s not a new finding:
I’m regularly receiving mails, with incorrectly decoded Umlaut/ special characters in mailbody.
These mails show up correctly in IMAP-(Apple Mail) and ZPush-(Outlook, iOS-Mail) clients. The issue seems to cover both, plaintext and HTML-contents.
In most cases, for those mails, the subject is rendered correctly (particulary since commit inetmapi: revert support for vmime RFC6532), but with a few rare exceptions: In these cases, the subjectline still shows up correctly in IMAP- and ZPush-clients, whereas WebApp displays wrong decodings in Subject and body.
I haven’t found any clue, yet, where the problem might arrise from, but collected a couple of affected mails, which I could supply privately as EML.