We are using redmine to create issues from emails as described here http://www.redmine.org/projects/redmine/wiki/RedmineReceivingEmails

If someone sends a bugreport from within our software the generated mail is formatted a bit with the wiki markup:

Exception message¶

Description:

Description added by the user

StackTrace:

a stacktrace

DebugInfo:

product related information (version, loaded dlls, environment, operating system

recently we upgraded from redmine 1.x to 2.1.2
Since than, the formatting is removed from the mail body and only the raw text gets into the description field.

In the ReceivingEmails documentation, I found this hint:

Redmine tries to use the plain text part of the email to fill the description of the issue.
If a HTML-only email is received, HTML tags are removed from its body.

However, I verified that the E-Mail is plain text only with no html-part.
How can I tell redmine to import my wiki syntax again?