What's the alternative? Outlook is an offline app that most employees (technical and non-technical) know well how to use. The only other alternative (for enterprise) seems to be IBM Notes and that one isn't necessarily better (and often less liked by employees in my experience).

I don’t understand why people don’t like thunderbird.

It’s not as nice as mutt but I feel like outlook users would be comfortable with it.

Outlook's integration of mail and calendaring (and to-dos) is very, very, very useful for business users. Thunderbird is only one piece of that.

Thunderbird has a calendar and to-dos as well (though I never used them personally, so I can't speak to their quality). Does Outlook do any special integration between them (outside of just having them in the same app)?

Yes. It uses SMTP as a messaging channel for calendar invite sharing - sending a meeting invite to someone (through the calendar) sends them an email, which their outlook client picks up and treats as a calendar invite. It also integrates with Active Directory.

Not quite SMTP, but yeah. It's whole normal transport system. Add on to that things like integration with conference call systems (eg: Skype) for meeting and dial-in setup, conference room scheduling (Room Finder), and it's honestly a really nice tool for business use. It does a lot of things that users really like.

I've used Thunderbird for years- ever since Evolution became unworkable. But evolution was very good too back in its day- more Outlook-y.