I switch between mac desktop, windows desktop, my smartphone, and pure web when reading and writing emails all day. Using IMAP, everything stays in sync -- one of the few wonders of the Internet. However, on the mac running Mail.App, I couldn't get my sent mail to store on the IMAP server for other to see. Finally, after googling a bit and testing it out a bit, I got it to work (with my Dreamhost IMAP servers). The short version says "highlight the Sent folder within my mailbox, then select from the menu bar: MailBox/Use this Mailbox for/Sent". Initially I was worry about this one because for some reason, the Sent folder for my first IMAP account never showed up under the account. Instead I just got one big "Sent" mailbox (icon=paper airplane) on Mail.App which seems to be displaying the Sent folder. So I couldn't do the "use this mailbox for sent" for my first account. Turned out Mail.App is just "smart enough" to move the first server stored Sent box out to the paper airplane mailbox. Once I set the second Sent box in the second account, both Sent mailbox/folder shows up under the big paper Airplane as sub folder.
Similarly, if you have multiple IMAP account, for each of them, do the "use this mailbox for" trick for each of their Sent, Drafts, Trash and Junk mailboxes. Then you will see top level "Inbox", "Sent", "Trash", and "Junk" mailboxes on the left hand side of Mail.App, each with subfolders underneath setup for each IMAP account.
I actually like this Mail.App layout. It groups messages by type and consolidate them from each IMAP account, if you want to see them consolidated.