When importing subscriber lists (via Subscriber Import, NOT via API), I have difficulty to import addresses with umlaut characters. Umlaut in the Name or other fields work fine. Just in e-mail addresses they are not accepted. The CSV file I'm importing is in UTF-8 with BOM (this is why the umlaut in other fields work). I know that generally it's not a good idea to use umlaut in e-mail addresses, there are many things that can go wrong with these. On the other hand, when I receive a list from one of my clients for import in Campaign Monitor, I will have to use them somehow.
Just to clarify, do you get an actual error message when importing them? Please contact us via support, with an example of the content you are importing, and we can check it out.