Client-server connections in SMTP
Client-server connections and connection directionality in SMTP differ from how you may be familiar with them in other protocols.
For example, in the SMTP protocol, an SMTP client connects to an SMTP server. This seems consistent with the traditional client-server model of communications. However, due to the notion of relay in SMTP, the SMTP client may be either:
• an email application on a user’s personal computer
• another SMTP server that acts as a delivery agent for the email user, relaying the email to its destination email server
The placement of clients and servers within your network topology may affect the operation mode you choose when installing a FortiMail unit. If your FortiMail unit will be operating in gateway mode or server mode, SMTP clients — including SMTP servers connecting as clients — must be configured to connect to the FortiMail unit.
Terms such as MTA and MUA describe server and client relationships specific to email protocols.