With the advent of the Exchange hosting industry, the term "Split Domain" was generated. This term signifies the method that allows companies to utilize Exchange server features as well as features from another server...such as a Linux POP3 server. The typical reasoning for a split domain is to save cost.
Please note: A split domain is generally not recommended as the peripheral negative impact is often more that the intended benefit.
With the ability to move the Exchange users onto another (typically free) server for email functionality, the cost savings can be a significant. For example, this allows your Executive and Sales Team to utilize the collaboration and mobile features of Exchange and place the "less critical" employees on the POP server, all while maintaining a unified firstname.lastname@example.org company format for email.
There are several disadvantages to splitting a domain. It is important to know as much as possible before proceeding with a split domain scenario.
Complicated Server Management - Since there are two servers (POP3 and Exchange Server) each server will need to know about the users on the other server or inter-company communication between the employees on separate servers will be impossible. Configuring and maintaining the functionality can be difficult and laborious.
No Complete GAL (Global Address List) - Because some of the users will be on the POP3 server (server #1) and some of the users will be on the Exchange server (server #2) users will not be able to send to users on the POP server using the GAL (Global Address List). This may cause problems with the instances involving communications to the entire company, or employees not on the domain server in a Distribution List.
Potential increase in Spam - Most spam filter configurations will automatically score an email where the sender and recipient are members of the same domain as more likely to be spam. This is due to the prevalence of "spoofing", where a malicious sender will generate an email with the same sender domain as the recipient to make that email look like it is coming from another user within the same company or organization. This results in the recipient being more likely to open the malicious email, which may contain a virus or other malware. In a split domain scenario, this extra spam prevention to fight spoofing needs to be disabled. This will increase the amount and frequency of spam making it through to a user's mailbox.
Limited Support - Because email is linear and there are so many points in the line with the new servers in the equation, it becomes very difficult to troubleshoot the original point of a problem.
Steps to Implement
Assuming that your MX records are pointing to your POP3 server...
1. Create a Primary domain, within your control panel (i.e. domain.com) and notify our engineer team that this domain will be part of a split domain scenario.
2. Create mailboxes on the Exchange servers.
3. Add a Domain Alias (exchange.domain.com).
4. Change the MX records for exchange.domain.com to point to our services.
5. Add an SPF record to your DNS settings. You can find the SPF Record settings Here.
6. Create all the mailboxes on the POP3 server.
7. Create a forwarding rule on the POP3 server for every user on the Exchange Server. i.e. forward email@example.com to firstname.lastname@example.org