"This is a multipart message in MIME format." - Outlook for Mac 2015

"This is a multipart message in MIME format." - Outlook for Mac 2015

Dear Sirs, 

We are getting loads of emails displaying "This is a multipart message in MIME format" and a very difficult to read body when moved to a folder other than the INBOX. 

This appears to be affecting only our Outlook for Mac 2015 IMAP connection to Zoho. 
On Outlook for Windows and Android all looks normal.

However, this is having a detrimental effect on our workfow and I would like to know if a solution is available please? 

Many thanks.