How do SMTP servers work?
You send mail from your email address, such as firstname.lastname@example.org, to a given recipient, such as email@example.com, using your email service provider. The webmail is known as the MUA, or message user agent.
The email is sent through port 25 to an SMTP server provided by your mail client, operating as a Mail Transfer Agent. The client and server start a brief conversation where the latter verifies all the information about the transmission of the message (sender, recipient, and domains). The mail server authenticates the email sender and places the new email in the recipient's inbox. Remember that the SMTP language only discusses the message's transmission and not its contents.
The email is immediately sent if the recipient’s domain has an account connected to the server. If that's not the case, the SMTP transmits it to another incoming server closer to the recipient. The domain server establishes a connection with the website server, which receives and stores the email.
What if the recipient’s server is unavailable or busy? The SMTP host simply forwards the message to a backup server. If none is available, the email is held, and the delivery is retried periodically. After a determined time, it marks the message as undelivered.
SMTP vs. IMAP vs. POP3
Specialized TCP/IP protocols called SMTP, POP3, and IMAP are used to send and receive emails. To meet the needs of modern applications, it's important to understand these protocols.
When utilizing the IMAP protocol, the email client establishes a connection to the server, scans for new messages, and saves them as temporary files in the cache. At first, the server only downloads the date, sender, and subject. Only when you open the mail will the content download. Therefore, it's possible to access the email's content without downloading any attached files utilizing this protocol.
The server updates when an email is edited, deleted, or has its status changed from unread to read. This procedure aids in reflecting the current status of emails across various devices.
The major differences between IMAP and SMTP include:
- SMTP is used to transfer data, while an IMAP server is used to retrieve messages.
- SMTP transfers data between servers, while IMAP facilitates communication between the server and client.
- While SMTP allows users to organize emails on client storage, IMAP allows users to manage emails on the server.
A more user-friendly approach to accessing mailboxes is through Post Office Protocol. POP3 allows you to access emails even if you're not connected to the internet because it sends emails from the server to the email client.
The client connects to the POP3 server when a user checks for new emails. The email client then submits its username and password to the server for authentication. The client sends text-based commands to retrieve all email messages as soon as it connects. It then deletes the server copies and disconnects from the server after saving the downloaded messages as new emails on the user's local system.
The differences between POP3 and SMTP include:
- POP3 is a pull protocol, whereas SMTP is a push protocol.
- Emails are delivered via SMTP from the sender's device to the recipient's inbox and via POP3 from the recipient's mail server to the recipient's computer.
- POP3 works between the recipient and the recipient's mail server, while SMTP works between the incoming and outgoing mail server.
POP3 vs. IMAP
The differences between POP3 and IMAP include:
- While IMAP displays the message header before downloading the email, POP3 downloads the email all at once.
- POP3 takes emails from servers, transfers them to a single computer, and deletes them. IMAP syncs the email across many devices so that it can be accessed via various channels after being stored on the server.
- Organizing emails in the mail server mailbox isn't possible with POP3, but it's possible with IMAP.