The null-character workaround only works with
ImapAuthentication.Plain and it actually makes the
Imap object use the standard method to authenticate/authorize to access a shared mailbox (as defined by RFC 4616).
If the following doesn't work, then it most likely means that Lotus Notes / Domino does not support this standard method or that it expects a different form of the mailbox string.
imap.Login(mailbox + "\0" + userName, password, ImapAuthentication.Plain);
Are you able to connect to a shared mailbox using any third-party IMAP client?