I seem to recall that there was an Office version for Sun Microsystem's Solaris, which is 99.99...% of the way to a Linux variant. That was killed.

(This may have been MSIE or an Exchange-compatible email client, I'm researching this still.)

OSX is better supported, though for a long time Microsoft's email client was not Outlook but Entourage, a now-discontinued project. It had a typical-for-Microsoft opaque binary data storage format, though it's proved possible to extract useful information from this using Linux utilities.

Microsoft discontinued MSIE support on Mac in 2005, an issue given that many enterprise Web / intranet tools relied exclusively on nonstandard MSIE web extensions.

Office was never offered for BeOS, which I believe was a deliberate strategy decision, though I'm not finding evidence of this (JLG should be able to comment, if anyone has current contact). The lack was seen as a kiss-of-death for the OS and hardware.

Alternatively, what made RIM's Blackberry as popular among business executives as it was was its integration with MS Exchange email servers.

Strategic control over what ports were and were not supported by Microsoft, regardless of technical difficulty or merits, was a major element of that company's monopoly abuses.