Express Plus

From Chaos Software Group:

Business email client with mail-merging for bulk personalized mailings and powerful spam filtering that eliminates messages on the server side before wasting your time downloading junk mail. Express Plus does not use a database to hold your messages, but instead stores each message as a unique individual file on your PC. Besides having no problem with database corruption, this method allows any active-scanning anti-virus program to work more effectively to prevent infection of your PC by cleaning, quarantining or removing messages with viruses before you or another user can click on them (and this doesn't require your anti-virus software to use any specialized mail add-ons to protect you). Express Plus can send personalized email to your contacts with mail merge templates you create (no more "Dear Friend" letters!) and can manage up to 250 different mail groups.

This program also offers shared mailboxes that can be opened by multiple users at the same time on a network. It supports multiple e-mail accounts in one mailbox, has the ability to read messages live on your server without downloading (using a feature called Server Headers), provides easy access to reading email headers, plain text version, html version, or the attachment list of an email, supports POP3, SMTP, and LDAP servers, and as an option, Express Plus can be combined with the powerful Time & Chaos information manager to offer a complete and integrated package.