I configured a special mail account, which writes each received mail to a triage project (id="dispatch").
All my projects have some mandatory fields, ex 'client' with default value.

The .forward command of the mail account is the following :

"|/redmine/rdm-mailhandler.rb --url 'URL' --key KEY --project dispatch"

Despite I don't allow overriding, and that a "allow-orride=none" option doesn't exists, I discovered that:

• the title of the email is scanned for demands IDs : if a title contains [text #ID], the mail is added as a note to demands #ID
• the content is scanned for fields values : if the content contains a line like 'client : bad name', the mail is rejected.

I doesn't have time to do more tests, but this seems to be a bug.

As your users may rely on this behavior, can you add a custom option to rdm-mailhandler like :

--dissalow-scan=title,content