Try claws or sylpheed (from which claws is derived), these work just fine on older hardware. Startup time (with cold cache) on my 14yo T42p is under a second, R

Try claws or sylpheed (from which claws is derived), these work just fine on older hardware. Startup time (with cold cache) on my 14yo T42p is under a second, RSS is 48MB with 4 active multi-gigabyte IMAP accounts. There is a bunch of extensions to add PGP integration etc. On Debian the package is called 'claws-mail', this name might be different elsewhere.