There is no out-of-the box method for this in Rebex IMAP, as the folder path and name for sent items might differ at different servers.
If you look at the RFC, it says:
"The case-insensitive mailbox name INBOX is a special name reserved to mean 'the primary mailbox for this user on this server'. The interpretation of all other names is implementation-dependent."
With servers that implement RFC6154, you could take advantage of the ImapFolder.SpecialUse property, but these servers are rather a minority. With other IMAP servers you are stuck with guessing from the folder name...