Deleting some Mail files

Hello,
I recently made the switch to an SSD (YAY!) and am trying to trim off extra files I don't need anymore. I ran OmniDiskSweeper and found that User/Library/Mail was over 5 GB. I use the Mail app to access 2 university email accounts. One of them is handled my gmail (IMAP) while the other is an Exchange 2007.
What would be the best way to trim this folder down?
Since these are both server based email accounts, I don't need attachments on my computer. Those I want, I download, rename, and place in the appropriate place.
I also don't necessary need all my older email messages. Is there a way to delete them from my computer but leave them on the servers? That way if I need an old email, I can still look back to find it and re-download anything pertinent.
Sorry to be long winded, but I am not exactly sure what I am asking as I am not sure what is possible. Any advice would be great so I could trim this folder down by a few GB.
Thanks,
RJ

 

TheGenerous, I am not sure I understand your post. Should I empty the mailboxes folder?
I did go into the Mail Preferences and change the download settings. For one account I have only download messages. For the other, I have have download nothing. So now much less new stuff should crowd this folder. But now I want to know what I can delete to claim back some of the 5GB. I don't think I need the old stuff and if they are still on the server, I can always go back if I am online. This computer primarily stays at home so I don't need much offline access to messages.
Thanks for your ideas,
RJ