From the description of your propose project it sound like something I have done. You mention that you have Exchange, Domino, POP and browser users. Not knowing what version of Domino you have I can tell you that it is the only messaging package that will server your needs for now and in the future.
Here is why:
Easy to use
Build in ACL, Access control List
Anti-Spam build in features
Any numbers of Anti Virus protections
Can use the browse for email, iNotes and your calendar & scheduling will ready and available to all users
POP Mail access via Outlook or any Oulook client, if you use the Extended mail template, meetingand Calendar are available via Outlook.
Native LDAP support for name resolution for POP users
The 6.5.x server and client can support a very large number of mail users. You also have and can use the WorkSpace as mini Portal. And the future of IBM Domino/Lotus is looking even better with the support for all open standards. Three years down the road you can migrate your user to the new messaging service from IBM with minimal investment.
Another good thing about this solution is that you have Collaboration build in into the client.
Well, good luck and you need a good messaging engineer let me know.