Although the title sums it up nice, I'll repeat and explain.

What would be a good email client for handling large amounts of mail ?

Large portion of mails I receive come with attachments (zip, rar, pdf, dwg, etc.) and within a month I usually have another 1,5-2Gb of new mail. I've noticed that 'standard' Outlook Express (with whose interface I've been very happy) gets awfully slow after a while. Archiving helps but not much. Then I usually take the files, move them onto a dvd, delete all messages I can do without and start anew. The thing is, I would love to have them all in email client since I often go after some old mails (slow projects).

So, what would be good alternatives ?

If it is portable, that would also be nice, but I can also live without it.

post scriptum: I love @gmail, but cannot use it for work. I know I could theoretically forward all of it there, and back, but that approach doesn't make my boss very happy (email handling policies and similar).