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updated: 6/17/98

 

 

 

Related Topic

Mail Service Primer


Mail Service Overview

SMTP (Simple Mail Transfer Protocol) is one of the TCP/IP protocols, used to send mail from one mail server to another mail server over the Internet or over a TCP/IP network. Clients connect to the mail server to send and receive mail using an SMTP-compatible mail client, such as Claris Emailer, or Eudora, or Microsoft Outlook Express.

Using the US Postal Service as an analogy, here's how a simple mail transaction works:

E-Mail
USPS Mail
User composes a mail in his mail client application Person write a letter
User addresses the mail with the recipient's email address Person addresses envelope with the recipients PO box and zip code.
User clicks "send", which causes the mail to be sent to his own mail server (the one set up in client mail software) Person brings the letter to his local Post Office for delivery.
If the mail is being sent to a user who has an account on the same server, the mail is placed in his mail folder, waiting for the client to connect to receive it. If the mail is being sent to that same zip code, the letter is put into addressee's PO box till he comes to get it.
If the mail is addressed to another mail server, the mail server makes a connection to that mail server to deliver the mail there. If the mail has a different zip code, the piece of mail is delivered to that Post Office, where it is placed in the addressee's PO box till he picks it up.

When clients send mail to the mail server for delivery, they also use the SMTP protocol. However, when clients receive mail, they use a different protocol, typically either POP3 (Post Office Protocol) or IMAP (Internet Mail Access Protocol).

POP3 downloads your mail immediately to your local workstation when making the connection. Unless other preferences are set, the mail is no longer stored on the server at that point. This type of connection is useful for users who connect from the same computer all of the time, and/or for users who travel and need to make a dial up connection to collect their mail. They can dial in, download the mail, disconnect from the server, and read, compose, save their mail as needed.

With IMAP, your mail is not downloaded to the client when you connect; it is kept on the server. Whenever you create, read, and manage your mail, you must have a connection to the server. This type of connection is useful when clients must connect from many different computers; since the mail is stored on the server, they can access it no matter what machine they connect from.

Planning your Mail Services

Client Software

The AppleShare IP Mail server supports SMTP, POP3 and IMAP. These are all TCP/IP protocols, which is the focus of this section.

When deciding on whether your clients should use POP or IMAP, consider the following:

    IMAP

    • IMAP is preferable when clients must connect from different worksations.

    • Because the mail stays stored on the server, it is preferable to have a LAN-connection to the server, as opposed to a dial-up connection.

    • Because all the mail is on the server, you should arrange for the mail database to be backed up regularly to avoid loss of mail should the hard drive fail.

    • Because all the mail is stored on the server, you should plan for an adequate disk capacity to store all your users' mail.

    • Because clients stay logged in, you should consider how the number of active clients will affect performance. ASIP can support up to 17 simultaneous IMAP connections with the default memory partition, and more if with additional memory allocated to the Mail Server application. But having a large number of users actively reading and sending mail on the server may affect performance.

    • Mail clients that support IMAP include Microsoft Outlook Express, Mullbery, and JavaSoft's IMAP client.

    POP3

    • POP3 is useful when your clients connect from the same machine; otherwise, when their mail is downloaded, it tends to be scattered across the various machines they connect from.

    • POP3 is useful when you connect to the server over a slow or expensive link. If your clients travel and have to dial in long distance to get their mail, its preferable to allow them to download all their mail, then disconnect to read, reply and compose.

    • When using POP3, mail is not stored on the server, so the client becomes responsible for backing up saved mail.

    • With POP3, users log in and out relatively quickly, so fewer clients tend to be connected at any one time. By default, ASIP supports up to 17 POP connections. This can be increased by allocating more memory to the ASIP mail server application.

    • POP3 may require less disk space on the server, since mail files are downloaded to the clients. Most POP clients have a a setting that allows users to keep their mail on the server, as well.

    • POP3 mail clients include Claris Emailer and Eudora.

How many mail servers?

  • How many ASIP mail servers you want to set up at your site will depend on the number of users you have, the layout of your networks and servers, whether you select POP3 or IMAP.

  • If you expect that your organization’s demand for mail service will result in your Mail Server approaching or exceeding the concurrent user connection limit, or the message storage limit (approximately one million messages), you might want to distribute your mail services across multiple computers.

    When mail service is shared among multiple computers, each computer that runs the Mail Server needs to store incoming messages for the users who connect to the current server, and forward incoming messages addressed to users who connect to the other Mail Server(s).

Mail Servers & DNS

  • When setting up DNS for mail servers, you create an "MX record" or mail exchanger, which indicates what machine handles mail for the host. The MX record may contain more than one machine to which mail can be sent; each entry has a preference value, with the lowest number indicating the highest preference. For example, your company, "acme.com" may have an MX record that indicates its first priority is for mail to be sent to the machine called "mail.acme.com" and if that one's not available, send it to "mail.ISP.com".

  • When a mail server needs to deliver mail to another mail server, it normally does a DNS lookup for the values in the MX record and attempts to deliver to those machines, starting with the most preferred host. So your mail servers will need DNS entries, which will require, for each mail server:

    • a static IP address
    • a host entry in the DNS server for your domain
    • an MX record, which indicates the name of the machine which provides mail service (which may be its own name), plus (optional) the name of an alternate machine to which mail could be delivered, as a second choice. This might be your ISP's mail server.

Relaying mail:

  • An alternative to having your ASIP mail server deliver mail directly to the destination mail server is to have it send all outgoing mail to some other SMTP server, and let it deliver the mail. For example, you might relay all outgoing mail to another SMTP server at your site, perhaps one that has Internet access, or you may relay it to your ISP's mail server.

Postmaster duties

The Postmaster is usually both the person who maintains the mail server and an email address to which problem mail (undeliverable) may be sent. The duties of the PostMaster include:

  • Setting up users, enabling mail, entering mail settings.
  • Verifying that DNS entries have been configured for mail servers

  • Testing that the ASIP mail server can both send and receive mail from other ASIP mail servers (if any), and external mail servers (if any).

  • Backing up mail server database, as needed.

  • Investigating problems with mail delivery. This might include helping users configure their clients so that they can send and receive mail. It might involve finding out why mail is not being sent or received.

It is not uncommon for the Postmaster to get access to other peoples' mail, either in the course of troubleshooting problems or handling undeliverable mail. Although reading other peoples' mail is unethical, users should be made aware that mail is not necessarily private, and the Postmaster duties should rest with someone completely trustworthy.

The ASIP Mail Help files and the troubleshooting section will assist you in Postmaster duties, by showing you the tools and techniques to maintain the mail server.