Outlook express has 2 GB limit on EACH ".dbx" file.Once you reach that limit,you cannot recieve and send any more messages.
If your E-mail messages reached 2 GB then that means you have TOO many E-mail messages.Probabably more than 100000.
Also some of your E-mail messages might have big attachements which take a lot of space.What you should do is to immediately delete all the messages and attachements you do not need because if you mess with those 2 GB DBX files,they can become corrupt and I am pretty sure you don't want that to happen lol.You can always compact them all in order to free some space,but I wouldn't recommend it.
What you should do is to create new empty folders and use THEM to send and recieve new E-mail messages.As for your old 2 GB DBX files,you should store them on some place safe so you can access them at any time when neccessary,but do NOT mess with them because if you do something wrong,they WILL become corrupt and inaccessible!2 GB is the limit per each DBX file so once you reach 2 GB,you cannot fill them with E-mails anymore and for that reason store those 2 GB DBX files somewhere safe and don't mess with them.Use them ONLY if you need to read some important old E-mails.
And for your NEW E-mails,create new empty folders on whatever location on the HDD you prefer and called them however you want then go to Outlook Express and then go to TOOLS->OPTIONS then go to the tab called MAINTENANCE and click the button called "Store Folder..." and there set the location for your new E-mails.
You can also use the SAME location for your new E-mails only in that case be sure that you move your old DBX files from that location on some other place or simply there create a new folder,call it let's say OLD DATA and move all your DBX files in that OLD DATA folder.On this way you can use your old location for new future E-mail messages (DBX files).Once they get full at 2 GB again,just move them on some place safe again and repeat the procedure.
If you have any questions,feel free to ask.
Cheers!