Running SMTP for POP and IMAP Clients

POP and IMAP protocols are used only for receiving mail; you must configure SMTP on the front-end server so that POP and IMAP clients can submit mail. You do not have to run SMTP on the Exchange front-end server. Instead, you can use another server as a dedicated SMTP gateway.
Important:
To run SMTP on the front-end server and enable it to accept inbound mail (mail for your domains), you must mount a mailbox store on the front-end server. This mailbox store must not contain any mailboxes. You must mount a mailbox store on the front-end server because any non-delivery reports (NDRs) must be routed through the mailbox store for formatting.
To configure SMTP so that POP and IMAP clients can submit mail to external domains, you must allow relaying.
By default, Exchange allows relaying only from authenticated clients. It is recommended that you keep this default. Clients such as Microsoft Outlook Express 6.0 and Microsoft® Office Outlook® 2003, and previous versions of Outlook Express and Outlook support SMTP authentication in addition to Transport Layer Security (TLS) encryption.
You should not allow relaying in either of the following ways:
• You should not allow anonymous relaying to all IP addresses; if your front-end server is connected to the Internet, doing this allows anyone on the Internet to use your server to send mail.
• You should not allow relaying from specific client IP addresses. Even if you are familiar with the subnet from which clients send mail, the Internet environment makes it difficult to determine such a specific set of IP addresses.
Note:
If you want the front-end server to act as the bridgehead server between your company and the Internet, it is recommended that the server on the Internet that accepts mail for your domains has the ability to scan incoming messages for viruses.
Note:
For more information, see the Exchange technical guide, Exchange Server 2003 Transport and Routing Guide.