Outlook can be a great corporate solution, especially since calendars and contacts can be shared. It's also nice because it is common (so most users don't need to be trained in it) and there's lots of documentation. Although many viruses do target Outlook, a properly admin'd network will have no problems (virus scanners for incoming/outgoing email, virus scanners on workstations, disabling scripting on workstations, etc).
[edit]Just a side note...security holes are USERS, not clients. Although there may be a weakness in the client software, generally speaking (and yes, there are very few limited exceptions), the *user* must do something to infect a machine: open an unknown attachment or one that's obviously a virus because of subject/body/name of file/extension of file. Educate users on top of scanning email (there are several engines out there that can be set to scan email going through an Exchange server and even alert sender/recepient that a particular email is infected), don't be afraid to shut down mail servers EARLY in the event you start getting hit by a worm (better 1 hour of downtime now than 6 hours in the middle of the night reconstructing major damage), and you should be just fine[/edit]
~Ladi