There's no question some clients are better than others at handling large quantities of email, but almost without exception we aren't aggressive enough keeping our inbox and other mailbox folders cleaned up.
Your inbox should be cleaned out completely daily. If things are really out of control (like me), maybe weekly at worst. Put email in organized folders that make them easy to get to. Even if you have to create a "not-yet organized" folder just to get stuff out of the inbox, do it. You can get back to that folder to go through it as time allows. In the meantime your inbox doesn't drag you down. Not only is inbox and other folder navigation faster, but the loading of your email client and the downloading of emails will be speedier as well.
I have discovered a VERY helpful utility that archives email, without making it too difficult to view and search them quickly and easily. It gets old emails completely out of your email client. This is important if you have huge quantities of emails that you simply must keep over the long term. The program is Mailstore. The free version is functional enough for most people (including me), works with all the most popular email clients and file formats. They offer a paid version with additional management capabilities that can archive email for almost any size small business. Here's a feature highlight:
- Archive and consolidate all email messages centrally, securely and permanently
- Search all your email within fractions of a second (incl. the contents of attached documents)
- Integrated CD/DVD burning utility to backup your archives with just one click
- To conserve disk space, MailStore Home saves only a single instance of each attachment and applies a smart compression algorithm
- Preview archived messages directly in MailStore and reopen them in your local mail client (e.g. Microsoft Outlook)
- In addition to backing up the entire archive, your messages can be exported to a variety of destinations
- Messages are stored MIME-compatible, and can be recovered at any time without information loss (e.g. to import them in other applications)