Recently, my unlock 3g iPhone running 4.2.1 with 6.15 stopped showing my email messages for my exchange email accounts. It goes through verification fine but when I try load my messages, it says it is updated but I see no messages. Only the inbox folder is showing with no messages inside. It worked fine up two days ago. I have made no changes to the phone since I unlocked it.
Any ideas and help will be appreciated.

Do the messages appear when you connect to the Exchange server via other means? (Outlook / Outlook Web Access / etc.)?

I've never heard of a jailbreak/unlock impacting Exchange specific connectivity. I'd find it easier to believe something happened with the e-mail server than something with the jailbreak caused it, but I suppose it's definitely possible. Did you install any Cydia tweaks that impact e-mail use?

Messages seem to appear fine in other venues. Actually I tested it in iPhone 2 and it worked fine. So,I realised there was nothing wrong with the server.
Yesterday, I was able to reload iOS 4.1 and everything seems to work fine again. I have 2 exchange accounts, both accounts have email, calendar and contacts. With iOS version 4.2.1 all stopped working suddenly. Everything was fine up to March 27th and then nothing was able to synchronise. In the meantime, my phone was the same, no new feature or application was installed. I had also an issue with Gmail and Yahoo mail. They lost synchronisation of email and calendar. I was able to get the email for Yahoo and Gmail to work by deleting the email accounts and then recreated them. The calendar, though, never came back to normal, no sync.
Right now, with iOS version 4.1 everything works fine. And I hope it stays that way.