If your company is connected to the Internet for e-mail (and it's rare that a company isn't), there is can be an incredible amount of "hidden" requirements in your company.
Consider the following table for some of the most common areas of consideration:
SMTP trafficCapacity planning for current SMTP mail traffic and growth are a given. Redundancy of links via multiple vendors, multiple DNS namespaces for different divisions of the company, division of SMTP traffic via dedicated inbound/outbound servers, and load balancing of connections are just some of the factors to discuss.
ApplicationsOther factors to consider are related to any Web applications that your company might be using that send data internally or externally via the email system. Consider dedicated virtual servers with appropriate security controls to isolate this traffic.
Client accessSecure access using SSL for POP3 and IMAP4 clients is a must as well as for Outlook Web Access (OWA) clients. Again, creation of dedicated virtual servers for this client traffic and perhaps each individual protocol might be merited in companies with large remote user bases.
As you consider access from the public Internet, consider any possible connections that your company might maintain to partners and vendors in an extranet configuration. This might include a shared virtual private network (VPN) for accessing specific servers and data within each other's networks or demilitarized zones (DMZs) between firewalls. You might want to place an Exchange 2000 server in the DMZ of your company for extranet usage.