I use Arie's solution (with Thunderbird) but have also set up two additional e-mail addresses "email@example.com" and "firstname.lastname@example.org".
The laptop is set to Bcc a copy of any e-mail I send to the desktop, and vise-versa. I then set up message rules on each machine to mark the Bcc'd mail as read and move it to the "sent" folder, that way I have a copy of all incoming and
outgoing messages on each computer.
My desktop runs 24/7 so it leaves a copy on the server, and I set up a rule on the laptop to delete any messages older than 30 days. I have corporate web hosting with massive disk space though, so I am lucky enough to not have to worry about filling it up with e-mail!
If you don't run the desktop 24/7, then the only safe way is to use the "leave a copy on the server" rule on each machine, then manually delete a load of e-mails once a month or so, when you know that both machines have downloaded the latest mail.
The Gmail IMAP or similar option is fine, but in my opinion it doesn't look very professional to not
use your own domain name in a business scenario.
Prcotter makes a valid point though, there are a lot of ISP's that have IMAP facility as well as POP3
, but they don't openly advertise it as it's encouraging customers to fill up ISP disk space... and that ultimately costs the ISP extra money!!