[E2010] [EWS] [C#] [Windows]: Exchange-service not working anymore after upgrade to 2010

I have searched for this error/question I have but the solutions were not related to my situation and/or didn't provide any further infiormation. That being said... Under Exchange 2007, following code was running properly:

ServicePointManager.ServerCertificateValidationCallback = CertificateValidationCallBack;
ExchangeService service = new ExchangeService(ExchangeVersion.Exchange2007_SP1);
service.Url = new Uri(ConfigurationManager.AppSettings["ExchangeWebService"]);
service.Credentials = CredentialCache.DefaultNetworkCredentials;

EmailMessage mail = new EmailMessage(service);
mail.Subject = subject;
mail.Body = message;
mail.Attachments.AddFileAttachment(row.Path);
mail.ToRecipients.Add(receiver);
mail.Send();

Now, there has been a migration to Exchange 2010 and I get the following message from the `Microsoft.Exchange.WebServices.Data.ServiceResponseException` that is thrown:

When making a request as an account that does not have a mailbox, you must specify the mailbox primary SMTP address for any distinguished folder Ids.

I know that the enumeration for the ExchangeVersion can be changed but when I change this to `SharePoint2010` for example I get following error-message:

Exchange Server doesn't support the requested version.

Are there settings in Exchange that need to be set for which I have to contact the Exchange-administrator or is there something in the code that needs to be changed?