IBM Domino 9.0.1FP4 HF523, IBM Notes 9.0.1FP5
We have a problem receiving incoming SMTP e-mails from a particular (external) sender that's only recently started, in that all formatting is stripped out of the message and arriving as plain text.
Looking at the page source in the Notes client there's only one MIME part, which contains just the bare text as displayed... no additional HTML parts or file attachments.
However if I enable saving of the incoming SMTP stream at the server with
set config SMTPSaveImportErrors=2 and look at the resultant text file, there's an additional MIME part with a winmail.dat file attachment.
Decoding this raw text and running it through a winmail.dat decoder reveals the original e-mail with all formatting intact!
Nothing has been changed to our server version or configuration recently, and although I'm unable to say the same for the sending server, the raw source of the message (presumably before Domino's tried any conversions) suggests the Domino SMTP server is screwing something up. It's as if it tries to convert winmail.dat, has a problem, then bins the attachment entirely.