I suspect this may have been caused by return of a large, undeliverable file. The message I get from my Mac (iBook running 10.3.9) and Mail (1.3.11) is:
I've been able to access my e-mail through my ISP and delete the undeliverable file, but still cannot open my In Box. Also, it will not allow me to "read only".
Any suggestions will be truly, truly appreciated. Truly.