Syncing Macbook and iMac with .Mac. Please help

Hi!
I've just bought a one year subscription to .Mac after trying it out for a while. So far i really like it. Except for syncing Mail.
Ive set the settings in both the macbook and imac to sync mail accounts, settings etc.. But some messages doesnt show up on both macs. I use the imac most and receive most of my emails on this one. But when I take the macbook to uni or on holiday, i would like it to get all the new mails which have arrived since last time it synced with .mac. Is that possible? If not Im hoping it will at least be able to sync all the new messages received on the macbook, when i go back to the imac. Anyone know if this should/will work?

Also, if anyone else knows anything I should know about .mac, please let me know

Thanks so much