Firefox and Thunderbird

• CAVEAT: Author uses Thunderbird occasionally (still tied to vm!)
  
  
• Thunderbird is a redesign of Mozilla Mail
  
  
• Integrates smoothly with Firefox; built-in Spell Checker!
  
  
• Faster, smaller memory footprint than Mozilla
  
  
• "Bayesian" Junk Mail Filter, "Junk" button; also Message Filters (example).
  
  
• Customizable toolbar, Extensions, and Themes (like Firefox), e.g. EnigMail
  
  
• Multiple Accounts are supported; IMAP/POP3 over SSL (or not, if you must)
  
  
• Security: Fine-grain control over javascript, images in mail; more secure than Outlook.

BUT...

• Thunderbird not "integrated" with calendar like outlook
  
  
• However, Mozilla Calendar (aka Sunbird) aims to meet this need (soon).