What I don't see in this list is:
1) budget
2) number of people in the office now
3) what kind of firewall you have.
It is possible that your DSL router will provide NAT which will offer some protection for your hosts but a firewall would be advantageous.
The number of people in the office is important since that may dictate how you setup the network - i.e. using NAT/PAT to share the line, provide static IPs to each user, etc. (I recommend using NAT/PAT anyway, since it makes growing the network easier).
What's your budget? configuring a mail system for that many users is certainly possible with free software that's either already installed in Mac OS X or is freely downloadable. However, maintaining 5000 users in a normal sendmail/popd configuration using the password file is a PITA. At the very least you should look at some kind of central user database. If your budget permits look at something like Stalker's Communigate Pro. It costs $$$, especially for that many users, but it'll be easier to setup and maintain, and includes SMTP, POP3, IMAP and WebMail interfaces.
1) budget
2) number of people in the office now
3) what kind of firewall you have.
It is possible that your DSL router will provide NAT which will offer some protection for your hosts but a firewall would be advantageous.
The number of people in the office is important since that may dictate how you setup the network - i.e. using NAT/PAT to share the line, provide static IPs to each user, etc. (I recommend using NAT/PAT anyway, since it makes growing the network easier).
What's your budget? configuring a mail system for that many users is certainly possible with free software that's either already installed in Mac OS X or is freely downloadable. However, maintaining 5000 users in a normal sendmail/popd configuration using the password file is a PITA. At the very least you should look at some kind of central user database. If your budget permits look at something like Stalker's Communigate Pro. It costs $$$, especially for that many users, but it'll be easier to setup and maintain, and includes SMTP, POP3, IMAP and WebMail interfaces.