ChemIT notes on specific situations where we have noticed Outlook outperforming Mail.
Specific situations ChemIT strongly recommends using Microsoft Outlook on the Mac
You have a reasonable amount of email.
Performance languishes with just several thousand email messages.
Behaviors: Slow, unresponsive, unreliable/ gets confused
You have administrator responsibilities
Outlook's calendar integration is often beneficial.
You use a Cornell "special mailbox"
Outlook handles it better than Mail does.