Though IMAP is quite powerful, it is also rather fragile. It is quite likely that you need to adjust the settings in order to get the IMAP server to recognise your log-in. Rest assured that Outlook is quite good at this - though only in the latest versions. Old versions of Outlook were notoriously bad at following IMAP conventions.
If you can provide some more information, I can update this answer with more specifics. The things to look for are:
- What security settings are needed? This is the most common problem. There are a number of different ways to secure IMAP servers, all with slightly different settings.
- What IMAP server type are you connecting to? e.g. Dovecot, GMail, etc. This makes a difference to the settings used.
- What are the namespace settings used on the server? This will probably be answered by the question above. IMAP allows folder use but there are a variety of ways that the folders can be configured.