Share a Mail.app mailbox between multiple computers I have a Mail.app mailbox on my Mac where I archive mails received from an IMAP account which has a relatively strict quota on mailbox size (500MB). I write mails into the archive once a month (as I reach my quota), but I read from it frequently. The archive mailbox has grown to a couple of gig in size. Having just bought a second Mac, I'd like to share this mailbox across Mail.app on both Macs. Please can some suggest a good solution for doing this? I am reasonably happy to ignore concurrent access problems as a) I will almost certainly never make concurrent writes to to the archive, and b) concurrent reads should be ok for most systems that I can imagine.

My first attempted solution was to move the .mbox file to Dropbox and soft link (ln -s) that file (actually a directory) back to its original location. However when I opened Mail.app the mailbox disappeared. Any suggestion on how to get this to work would also be appreciated.
I had a similar problem a few years ago and decided to move to a better email service. I chose Gmail, mainly because of the >10GB storage. 

You can migrate your locally stored messages via IMAP. Once you set up your Gmail account, add the IMAP server to Mail. Then drag and drop all the messages you want to migrate on the Gmail inbox. IMAP will do the rest. 

Adding the IMAP server to multiple devices will obviously keep them all in sync. 

In general, I would recommend this over using a local option. IMAP is a superb protocol and I don't see the advantage of trying to sync the local archive in any other way.