Fair enough. I'm curious then. What do you use for email?
SMTP / IMAP. that's all.
As for software, it's usually CommuniGate or Zimbra on the servers. For clients, I support Thunderbird, apple's Mail.app, kontact, Eudora, and the included clients on iOS and Android and even Blackberry. all of them work perfectly together when using standard protocols.
There are a couple of big bosses that insist on using Outlook, even though they can't say a single reason to prefer it, it's usually just "i'm too old to learn something new". One of them creates by himself easily more trouble than the three companies where i'm involved in support. What seems to be helping a little is that we created an isolation server running Fetchmail to keep a copy of all his mail and accessed exclusively from his one Outlook machine. all his other devices (two iPhones, an iPad, one Blackberry and a MacBook) work without a hitch directly on the CommuniGate server.