I created a campaign in HTML and tested a delivery.
When analyzed the result in Preview and click in More:
Seeing the tab Deco-mail diagnostic, there are many warnings:
I saw in training that this warning refers when to at least one customer in recipient whit field “Email Format” equal to Deco-mail or Decore Mail or Decoration Mail, but when we filter the recipient we don’t have any customer with this email formats.
Cause of your problem/feature: It's happening because in the Adobe campaign we have one OOTB typology rule to ensure that your email respects the format of all the recipients available in the target query. This particular typology rule, Deco-mail check, is started during the analysis and This typology rule is only executed if at least one of the recipients is configured to receive emails in Deco-mail (DoCoMo), Decore Mail (Softbank) or Decoration Mail (KDDI AU) format.
Ideally: You can create a workflow that contains two deliveries: one to be read on a Japanese mobile and another for recipients with a standard email format.
If this is something, you do not want you can disable this rule.