Can a Domino server be used on a 100 kbit/sec local area network with Cisco routers and a Microsoft exchange server and a Novell 5.0 mail client?
A 100 kbit/sec network would be pretty slow. ArcNet was 2MB/S and 10BaseT Ethernet is 10MB/s, so I'll assume you mean a 100 MB/s network with Cisco routers, an Exchange server and a Novell 5.0 mail client...
I don't know about the mail client - I've never used it (I am the development Expert, not the interoperability expert, after all <g>), however, if the Novell mail client can get mail from a mail server in either IMAP or POP3 format, it should be able to retrieve mail. You'll need an iNotes or iCal user ID license to (legally) retrieve it from a non-Notes client, though.
As far as Exchange is concerned, there are lots of examples of even large companies who have Domino and Exchange co-existing on their networks, so I don't see that being a problem directly.
Cisco routers are what 80% or more of the internet is running on, and I can get to my domino server over the internet when I'm out of town, so I don't think that'll be an issue, either.
So, in conclusion, unless you have a very specific problem that's causing some grief, the situation described above (with a 100 MB/s fast ethernet local areA network, etc, etc,) should work fine.