Performance degrades in almost all mail clients when there are many, many emails in one folder. Here are MS Outlook's limitations.
Info below announced at 11/21/13's CIT presentation on upcoming Office 365 upgrade.
Expect performance degradation in Outlook when surpassing these limits:
- 10,000 messages in any one folder
- 5,000 calendar entries
ChemIT staff have noted no performance issues with someone having over 40,000 message in their Outlook Inbox (Windows 7), but at least now we've been warned. :-)