My problems with the email client (I use IMAP and Exchange accounts):

• Does not show recipient list when viewing messages. (You must use "Message Details".)
• Does not download the body of messages for instant reading later. WM devices, for example, let you precisely control how much of a message will get downloaded, whether attachments are included, etc., and for 90% of my messages, they are completely downloaded by the time I am notified about them.
• Does not allow you to read some messages while it downloads other messages. For a multitasking device this is disappointing.
• Has phantom "xx unread messages" status indicators which do not go away even after you read the messages. No amount of send/receive syncing or re-reading the messages makes any difference. Seems like cache corruption -- goes away on its own sometimes but recurs often.
• Does not allow you to specify individual folders to sync or not sync. (I think there's a workaround by temporarily using another IMAP client though.)
• Does not allow you to edit a forwarded message.
• Does not allow you to open Inbox sometimes -- you have to sometimes race with the client to get it to let you open Inbox before it starts syncing, and then it locks the Inbox out.
• Does not handle Exchange calendars well -- I've had entire recurring appointments deleted from my Exchange server by my N900, and other times I've had appointments stay on the N900 and then get duplicated both locally and on the server -- hence every appointment gets doubled. My advice: Don't manipulate calendars with the N900 client; just sync the N900 with the server, and use Outlook Web Access to make any changes with the N900.
• Cannot accept/tentative/reject/propose new times for appointments.

To me, the email/Exchange client is the biggest limitation of the N900, more than all other issues combined.