1. The sender uses a Mail User Agent (MUA) to compose an email. An MUA, often referred to as a mail client, is a program that allows a user to compose, send and receive email. See a list of mail clients and how to configure them.

2. The mail is sent to a Mail Transfer Agent (MTA) which is responsible for sending the email to the receipient’s MTA. An MTA transfers mail messages between computers via the SMTP protocol. Postfix, Sendmail, Exim and Qmail are examples of an MTA.

3. The recipient’s MTA receives the email and passes it on to a Mail Delivery Agent (MDA). An MDA manages the user’s mailbox and handles mails for delivery to the MUA using either the POP or IMAP protocol.

   • POP (Post Office Protocol)
     Although most mail clients have an option to Leave a copy on the server, POP is generally used to download all messages from the mailbox, store them on the user’s PC as new messages and delete them from the server.

   • IMAP (Internet Message Access Protocol)
     Allows users to view their mailboxes on the server and to delete mail only when told to do so.

   Mbox and Maildir are two common mailbox formats used in Unix. Dovecot, Cyrus and Courier are examples of an MDA.

4. The recipient uses an MUA to check and retrieve messages from the MDA.

Your IP addresses are not blacklisted

Even a newly setup mail server could already be blacklisted. Some of the reasons are:

• For those using a dedicated server, the IP address given to you might have been used by a spammer in the past
• For those using a shared server, a fellow user might have sent a spam mail.
• For those using NAT to share internet connection, one of your users might have sent a spam mail due to virus infection

To check if your IP address is blacklisted, use
http://www.mxtoolbox.com/blacklists.aspx

Your mail server has an MX record

An MX record or Mail exchanger record is a type of record in the Domain Name System (DNS) specifying how Internet e-mail should be routed using the Simple Mail Transfer Protocol (SMTP). Contact your DNS provider to add an MX record for your new mail server. To check if your mail server has an MX record, use http://www.dnscolos.com.

Your mail server greeting matches your hostname

To test your mail server greeting, use http://www.dnscolos.com. If your mail server has multiple IP addresses, see Postfix Multiple IP Addresses Individual SMTP Greeting for a guide on how to fix this issue.

Your mail server is not open relay

An open relay mail server allows anyone to send mail outside your network. This means any Tom, Dick and Harry can use your mail server to send spam. To perform an open relay test, use http://www.dnscolos.com. If it fails, double check your Postfix Dovecot SASL configuration.

Your mail server has a reverse DNS

A reverse DNS returns the hostname given the IP address. Most mail servers will check if you have a valid reverse DNS, otherwise your sending reputation will be downgraded. Contact your internet service provider to add reverse DNS entries for your mail server IP addresses. To check if your mail server has a reverse DNS, use http://www.dnscolos.com.

Create a SPF DNS record if possible

Sender Policy Framework (SPF) allows software to identify messages that are or are not authorized to use the domain name based on information published in a sender policy of the domain owner. Not all DNS provider supports SPF so choosing a DNS provider that can do this is a plus. To check if your mail server has an SPF record, use http://www.dnscolos.com.

The Mbox Format

This is the traditional way of storing mail messages in the Unix world. In this format, a regular text file which serves as the mail user’s mailbox file is created.

Fig. 1: Mbox storage format

How Mbox works

Receiving and storing a mail

1. Lock the mailbox.
2. Append the header (usually “From [sender's email address] [date and time received]“) and the mail into the mailbox file.
3. Unlock the mailbox.

Retrieving a mail

1. Lock the mailbox.
2. Locate and read the mail.
3. Update the mail status flag.
4. Unlock the mailbox.

Deleting a mail

1. Lock the mailbox.
2. Move the contents of the mailbox, beginning from the position right after the mail to be deleted until the end of the mailbox, into the position of the mail to be deleted.
3. Reduce the size of the mailbox file by the size of the deleted mail.
4. Unlock the mailbox.

Searching a mail

1. Lock the mailbox.
2. Search the mailbox.
3. Unlock the mailbox.

Advantages

• Format is universally supported.
• Appending a new mail into the mailbox file is fast.
• Searching text inside a single mailbox file is fast.

Disadvantages

• Has file locking problems.
• Has problems when used with network file systems.
• Format is prone to corruption.

The Maildir Format

This is a new way of storing mail messages. In this format, a directory usually named Maildir is created for each mail user. Under this directory are three more directories named new, cur and tmp.

Fig. 2: Maildir storage format

How Maildir works

Receiving and storing a mail

1. Create a unique file in the tmp directory.
2. Write the mail into the newly created file.
3. Move the completely written mail into the new directory.

Retrieving a mail

1. Locate and read the mail.
2. Move the mail from new into the cur directory and append the mail status flag into the filename.

Deleting a mail

1. Delete the file containing the mail.

Searching a mail

1. Search each and every mail file.

Advantages

• Locating, retrieving and deleting a specific mail is fast.
• Minimal to no file locking needed.
• Can be used on network file system.
• Immune to mailbox corruption (assuming the hardware will not fail).

Disadvantages

• Some filesystems may not efficiently handle a large number of small files.
• Searching text, which requires all mail files to be opened is slow.

A partition is a subdivision on a hard disk. Each Linux partition except the swap partition requires a mount point.

Why should you partition your hard disk?

• Control data size. By separating the partition for the system files and user or application data files, you can prevent the user or application from using up all available disk space and impede the normal operation of the system.

• Compartmentalize storage. By using multiple partitions, in case file system corruption occurs, damage will be isolated to the partition of the corrupted file system only.

• Improve system performance. By using multiple partitions, you will restrict your hard disk access to a smaller area of the hard disk giving better performance.

• Run multiple operating systems. You can run multiple operating systems in one computer, each operating system having its own partition.

What is a mount point?

A mount point is a local directory on your Linux system where a partition or device is attached to. All partitions created during Linux installation will be automatically mounted.

This is similar to the mount point in Windows NT, which allows you to attach a new partition into an empty directory say C:\Program Files instead of assigning a new drive letter to the new partition. Mounting to local directory allows you expand a local directory by adding a new hard disk instead of replacing it.

What is a logical partition?

By design, you can create up to four primary partitions only irregardless of what operating system you use. To create more than four partitions, they have to be created as logical partitions within an extended partition. An extended partition is a primary partition which can be divided into multiple logical partitions. A primary partition is one of the four main partition that can be applied to the hard disk. A logical partition is a subdivision of an extended partition.

What is a swap partition?

This is a special non-user accessible partition in Linux used to store inactive applications to free up RAM. This gives the Linux system virtually more memory allowing more applications to be run. For Windows users, this is similar to the virtual memory paging file. Generally, the swap partition should be twice the amount of RAM you have.

What is a logical volume?

Logical volumes are a subdivision of a volume group. A volume group is composed of several partitions formatted as a physical volume. Logical volumes allow administrators to grow or shrink logical volumes without destroying data.