So, I've been using my Samsung Omnia for over an year to collect messages from my GMail account using POP. But, from time to time, something strange happens, and my Outlook or whatever Windows Mobile default mail client is called, suddenly stop retrieving e-mails. Connection is fine, logging is fine, retrieving is ..not fine.
So, the workaround for this, although you can loose some e-mails in your WM client, but don't worry, you'll have them all in your GMail interface (well, not actually loosing from your mobile, ..you'll just have some e-mails on GMail that will not get fetched to your smartphone), is that, when you actually notice that no e-mail is being retrieved anymore, go to GMail account > Settings > Forwarding and POP/IMAP and select "Enable POP for mail that arrives from now on" and click on "Save Changes". Then, go to your WM smartphone and click on "Send/Receive", and suddenly, it works again. And, of course, you'll not retrieve the e-mails from when it started to fail to work, till the moment you checked that radio button on GMail interface. That's all!