What is mail spool

A mail spool is a file or directory that stores email messages on a mail server or email client. It's a repository of email messages that are waiting to be delivered to their final destination.

In the context of email, a mail spool is also known as a mail queue or mail backlog. It's a temporary storage area where email messages are held until they can be processed and delivered to their intended recipients.

Here's how it works:

1. When an email is sent, it's stored in the mail spool on the mail server or email client.
2. The mail server or email client periodically checks the mail spool for new messages and processes them one by one.
3. Each message is checked for errors, formatted correctly, and then sent to its final destination (e.g., the recipient's email server or inbox).
4. If there are any issues with a message (e.g., the recipient's email server is down), it will be stored in the mail spool until the issue is resolved.

Mail spools can be used in various email systems, including:

1. Mail servers: Mail spools are used to store email messages on mail servers, such as Sendmail, Postfix, or Exim.
2. Email clients: Some email clients, like Microsoft Outlook or Mozilla Thunderbird, use mail spools to store email messages locally on the client machine.
3. Email gateways: Email gateways, which are used to filter and route email messages, often use mail spools to store messages temporarily before forwarding them to their final destination.

In summary, a mail spool is a critical component of email systems, allowing email messages to be stored temporarily while they're being processed and delivered to their intended recipients.