While this section addresses sendmail directly, most of the concepts should apply to any mail delivery system.
A sendmail system will probably be the most likely to be network bottlenecked, but that bottleneck will also most likely be outside the system itself. There are no special tricks since a typical sendmail system will be fairly balanced against typical hardware. Depending on the size of the mail store, some type of RAID system may be implemented, but the mail store data is very valuable so use a RAID 5 or other fault-tolerant option. If the store does get very large, you might also investigate the benefits of multiple caching layers down to the card.
Sendmail itself is very dependent on DNS lookups for its operation, and may be doing ident lookups as well. As such, some tricks on the network can really help. Linux 2.4 kernels offer a lot of QoS features, these can be used to prioritize DNS and ident lookups over bulk traffic, preventing blocking of POP, IMAP, and SMTP waiting on a DNS lookup. I would also strongly suggest implementing a DNS cache locally, maybe even on the sendmail server itself.
Memory and processor requirements will depend on the number of usersand whether you are providing POP or IMAP services, or only SMTP. POP and IMAP require logins, and each login will spawn another POP or IMAP process. If concurrent usage of these services is high, watch for OS paging.
Using POP for delivery has a very low memory and hard drive footprint. In POP, e-mail is typically downloaded from the server to the client. The POP server merely handles authentication and moving the data to the client. When a mail message is sent to the client, it is typically removed from the server.
Using IMAP puts much more stress on the system. IMAP will copy an entire mailbox into memory, and perform sorting and other functions on the mailbox. When a client requests a message, the message is sent to the client, but is not removed from the server. So all mail messages remain on the server until the client explicitly deletes it. The advantage to this is a user can have IMAP clients on multiple machines and still be able to access all the messages. Since most of the processing is offloaded to the server, this makes IMAP a great mechanism for light clients or wireless devices that do not have much memory or processing power.
Since the data on a POP or IMAP server are critical, you should be using RAID-5 or RAID-1 to protect the data and keep the server running. POP has low memory requirements, while IMAP has very high memory and disk requirements, and both will likely increase as more users are introduced to the system and they start working with mailboxes in the thousands of messages.
In either case, using SSL on top of POP or IMAP will also increase the CPU load of the protocols, but will have minimal affect on disk or memory usage.
It is not advisable to put a shell account on the same machine as one running IMAP or POP, due to the CPU and memory used by these protocols. Nor is it advisable to use NFS to export a mailbox to a shell machine, as NFS locks can cause corruption of a mailbox if sendmail tries to deliver a message and the user is deleting another message from the same mailbox.