One of my customers uses a email branding tool on top of exchange, which adds a specific html layout to outgoing messages.

the messages that customer sends me, arrive most of time correctly formatted as expected.
Some however are shown in zimbra webclient as ascii messages with the layout files in attachment.
The customer has created those messages no different than the correct ones.

I deduplicated the customers outgoing mail to an gmail-account.
in gmail the "broken" messages are correctly depicted using the layout.

So it seem an issue with Zimbra webclient?
The source text of the troubled messages look the same to me in gmail as in zimbra, but I do not know that format well.

Within a month I should have found the time to upgrade to zimbra 8, perhaps that would solve it.