Exchange Server 5.5 & SMTP

SMTP
The Simple Mail Transfer Protocol (SMTP) is the standard protocol used for sending and receiving Email on the Internet. The "Internet Mail Connector" (IMC) or "Internet Mail Service" (IMS) as its now called, which come with Exchange server support this protocol. SMTP is meant for permanently connected mail servers, it is a send/put protocol, there is no fetch/get functionality only a passive receive channel for another mail server to send to. Implementation over a dial up connection does not fit well with using SMTP for receiving mail and needs special consideration: -

• The ISP will have to give you a fixed IP address.
• When your ISP registers your DNS name it needs set up an MX and an A record on there DNS server, for your domain name and host. They should also register one of their email servers as your companys email server. Email addressed to anyone in your company is automatically sent to and stored on your ISP s email server ready for when you connect.
• You will have to agree on a method to dequeue your mail from the ISP's SMTP server when you dial in.

RAS/DUN
Exchange Server can be configured to use NT4 Remote Access Service or Dial Up Networking (RAS/DUN) via an ISDN TA or modem. You can schedule regular connections, and via dequeue commands trigger the remote SMTP server to send you incoming mail. Unfortunately the dial-up connections from Exchange are of a fixed time duration, so a message that takes longer than that amount of time to transfer will never be delivered - the sending host would keep retrying delivery until it eventually was returned as undeliverable. Yes most Email clients will allow you to break apart large attachments into multiple smaller messages that would get through this limited connection time, but my experience is a lot of users don't know how to configure this option, also it would still take many fixed time connection periods to retrieve the entire message.
You should configure Exchange Server to send all outgoing mail via your ISP's SMTP server, rather than using DNS to resolve each domains mail server directly as this will result in much shorter dial up connections.

DIAL UP ROUTER
By using a dial-on-demand separate router you can get round the problems of fixed time duration calls, when using Exchange Server's dial-on-demand RAS/DUN. Exchange Server will believe it is permanently connected. As long as your happy with out-going mail being sent immediately and thus invoking a connection to the Internet, the only problem you will have is how to arrange for the regular collection of mail. To force regular connections and dequeue commands to your ISP's SMTP server for the reception of incoming mail, you will need to use a separate NT scheduler to send a dummy mail messages, or issue dequeue commands to your ISP's SMTP server directly.
Scheduling the sending of mail rather than sending immediately, to control your ISDN line usage is a lot more complicated. You have to start and stop the "Exchange Internet Mail Connector" or "Exchange Internet Mail Service" in the same script (*.bat) file as the dequeue command above for receiving mail. See http://www.swinc.com/resource/exch_smtp.htm for details.

Exchange Server & POP3

POP3
The Post Office Protocol Ver-3 (POP3): - This protocol is designed to allow single users to retrieve mail from a server. It is used extensively by mail clients for the retrieval of mail due to the growth in the dynamic allocation of IP addresses by ISP's. Since it retrieves mail proactively in a fetch/get action, it doesn't require the client to have a fixed IP address and works well over dial-up intermittent connections. Note the sending of mail by these POP3 clients is still via SMTP. A normal POP3 account usually only contains mail for a single user name. Exchange Server doesn't support the use POP3 as a means of mail retrieval from your Internet ISP as standard, but it is available as a third party add on. If youre a small company not requiring a permanent connection to the Internet, then a POP3 gateway for Exchange will give you more control over your dialup connection. In order to be able to use POP3 for Exchange you must use an ISP who can provide you with a Global POP3 account for all your domain mail.

Virtual Motion Internet MailBridge for Exchange
Virtual Motion's has now been merged and become Acotec Advanced Telecommunications. Microsoft has recently licensed the Internet MailBridge POP3 gateway for Exchange for Small Business Server 4.5. It allows full control of when to send mail and how often to retrieve mail, either via NT RAS/DUN or a separate ISDN router.

OTHER RESOURCES

Microsoft Exchange Server home page
http://www.microsoft.com/exchange/

An excellent resource with detailed descriptions of dequeuing techniques with Exchange Server
http://www.swinc.com/resource/exch_smtp.htm

Virtual Motion Internet MailBridge for Exchange Server
http://www.virtualmotion.com/products/mailbridge_for_exchange.htm