New mail server and empty mail content - winmail.dat [closed]

I have two IMAP accounts in Outlook 365. If I send an e-mail from my 1st account to 2nd account the e-mail is received I can see the subject but the message body is missing. Instead I can see that there is an attachment but can't view it either. If I open the given e-mail in other client e.g. Rainloop (web interface) I can see the e-mail content and even the attachment - winmail.dat.

I know that Outlook can send the e-mail in RTF format in which case it uses this winmail.dat file. Some e-mail clients have problem to view it, however I'm surprised that Outlook has problem to view its own e-mail sent by itself .

What is interesting that this issue happens only with this 2nd account - e-mail server on Ubuntu (postfix, dovecot, ViMbAdmin). So apparently the server is somehow changes the e-mail so Outlook can't read it. I'm the admin of this server but I'm not an expert.

I was not able to find any article which helps with this - most of them are about disabling the RTF format of Outlook and use HTML/Plain text.

UPDATE:

I noticed that Outlook has problem only with e-mails formatted as HTML, the Rich formatted e-mails are displayed. Further I checked the plain/original version of the e-mail on the server. What is interesting that HTML version does not contain any winmail.dat attachment (the Rich version contains). However if I view the e-mail via Rainloop (even via Outlook) it displays a winmail.dat attachment - smaller than the Rich version of e-mail (~half size).

So it looks like that dovecot or its subsystem (or something else?) converts/sends the e-mail to client with winmail.dat...

Any help is appreciated. Thanks!

Solution 1:

Well, this was an Outlook issue. If I selected the To address from the auto-complete list, Outlook messed up the e-mail and could not render...