Please try using slash characters ('/') instead of backslash characters ('\').
Also, make sure that you are using Rebex Secure Mail 2014 R2 or later (or let me know if you absolutely can't upgrade for some reason).
These forms are supported in 2014 R2 (and both are equivalent):
imap.Login("Domain/Username/AliasOfSharedMailbox", password);
imap.Login("Username@Domain/AliasOfSharedMailbox", password);
(Where imap
is a connected instance of Imap
object and password
is Username's password.)