Question

Using the Exchange Web Services API, is it possible to determine whether a mailbox/e-mail address such as [e-mail] exists within an organization?

If so, which is the simplest way to do this and is it possible without the use of impersonation?

Case: A Windows Service regularly sends e-mails to people within the organization. It does not have any explicit knowledge about their e-mail adresses. It only knows their username and assumes that their e-mail address is [e-mail]. This is true for all users except for a few that do not have mailboxes. In these cases, it should not attempt to send the e-mail in the first place.

Solution:

As suggested by mathieu: look for user and e-mail address in Active Directory instead. This function gets the job done:

using System.DirectoryServices.AccountManagement;
// ...

public static bool TryGetUserEmailAddress(string userName, out string email)
{
  using (PrincipalContext domainContext = 
    new PrincipalContext(ContextType.Domain, Environment.UserDomainName))
  using (UserPrincipal user = 
    UserPrincipal.FindByIdentity(domainContext, userName))
  {
    if (user != null && !string.IsNullOrWhiteSpace(user.EmailAddress))
    {
      email = user.EmailAddress;
      return true;
    }
  }
  email = null;
  return false; // user not found or no e-mail address specified
}

Answer 1

Determining if an user has a mailbox with EWS only could be more complicated than expected, especially without impersonation.

If you’re in an Active Directory domain, you should rely on the DirectoryEntry information to determine the mailbox of an user, and send email accordingly. If you got your user login, it’s really easy to get the associated DirectoryEntry.