Sending email is one of the most common tasks online, whether for personal or business communication. Behind the scenes, email relies on a protocol called SMTP (Simple Mail Transfer Protocol) to send messages across the internet. But what exactly is SMTP and can it directly send emails by itself?Key Takeaway: SMTP is the protocol that defines communication between mail servers to send and route email messages. By itself, SMTP cannot send email – a mail client and server are needed to compose the email content and use SMTP to transfer it between servers to the recipient.
How SMTP Works in Sending Email
SMTP is a set of communication guidelines that allow mail servers to send and receive email messages. Here is a simplified overview:
- A mail client (e.g. Gmail, Outlook) is used to write and compose an email message
- The client connects to an SMTP mail server provided by the email service
- The client uses SMTP to send the email content to the SMTP mail server
- The SMTP mail server determines the recipient’s mail server address
- It opens an SMTP connection to the recipient’s mail server
- The message is transferred using the SMTP connection
- The receiving SMTP server delivers the message to the recipient’s inbox
So in summary, the client composes the email while the mail and SMTP servers handle transferring the content between services behind the scenes. SMTP itself does not send the actual email – it facilitates the transfer of messages composed on a client over the internet.
Components Needed to Send Email
To send email, the main components involved in the process include:
- Mail Client – The software or application used to compose, read and organize email messages. Examples include Gmail, Outlook, Apple Mail.
- SMTP Server – A server that connects to other mail servers to send and relay outgoing mail using SMTP protocols. It does not compose messages.
- Mail Server – Receives, stores and delivers the messages to mailboxes. Popular mail servers include Exchange, Google Mail, IBM Domino.
- Internet Connectivity – Email relies on internet connections to communicate between servers and clients.
The mail client creates the email content, then uses its designated SMTP server to transfer the content through various mail servers until it reaches the recipient’s mailbox on their designated system.
Key SMTP Communication Commands
SMTP uses client-server architecture to define the communication process between source and destination mail servers:
- The client SMTP server initiates a connection and session with the destination server
- Commands are issued back and forth to authenticate, send the message, confirm receipt
- Then the SMTP session is ended
Some key SMTP commands include:
- EHLO – Identify client SMTP server to the receiver
- AUTH – Authenticate through login credentials
- MAIL FROM – Specify sender email address
- RCPT TO – Specify recipient email address
- DATA – Send contents of message
- QUIT – Close the SMTP session
These SMTP commands allow the two servers to transfer emails between certified parties before securely ending the connection.
Can Humans Use SMTP Directly?
SMTP is mainly designed as a server-to-server protocol – not meant for direct use by human users. However, technically advanced users can manually connect to SMTP servers using tools like Telnet or OpenSSL to send raw SMTP messages.
But there are significant challenges around properly formatting messages, manually implementing encryption mechanisms like SMTP SSL/TLS, authentication etc. So it is complex for the average user to send SMTP mail directly.
Advantages of SMTP Protocol
Some key advantages of using the SMTP protocol for email delivery include:
- Universal – SMTP is the standard protocol supported by essentially all public email service providers and networks. This universal compatibility lets any SMTP mail server communicate with any other on the internet.
- Interoperable – Servers and clients from different vendors and technologies can exchange emails seamlessly with SMTP protocol acting as the common language to transfer messages reliably between disparate systems.
- Flexible – SMTP handles email transfer tasks while leaving the composing and mailbox storage to specialized mail client and server applications respectively. This separation of concerns provides flexibility.
- Reliable – Has resilience features such as retries, timed responses, validation checks to function reliably even when network issues occur.
These SMTP benefits have helped email remain a convenient, universally accessible and efficient communication mechanism for the last few decades.
Disadvantages of SMTP
However, SMTP does have some downsides:
- Not encrypted – By itself SMTP has no encryption which allows emails to be potentially read or modified by attackers monitoring traffic. Requires additional security mechanisms like SMTPS.
- No user authentication – There is no user login or check at SMTP protocol level, increasing spam risk. Falls to mail servers to add authentication.
- Text-based – Only supports sending simple text-based messages. Cannot transfer complex or multimedia data. This is handled by mail clients instead.
- Stateless protocol – No session history is maintained between messages. Each transaction is independent and irrelevant past communication.
Organizations often use supporting mechanisms related to encryption, security policies and spam filters specifically to mitigate these types of issues commonly associated with raw SMTP.
Main SMTP Security Extensions
In response to growing security threats, SMTP extensions have been developed to enhance the protection around email:
- SMTPS (SMTP over SSL/TLS) – Provides encrypted connections between MTAs using SSL/TLS protocols to prevent email snooping. Now widely adopted with most major email providers.
- SMTP Auth (SMTP Authentication) – Allows clients to authenticate to servers before sending emails using mechanisms like passwords or digital certs. Improves spam control and non-repudiation.
- SMTP STARTTLS- Adds encryption layer to communication allowing confidentiality and integrity checking of data. Ensures protection even if encryption not initially enabled.
These extensions bolster SMTP security for modern internet communication.
In summary, SMTP (Simple Mail Transfer Protocol) is the fundamental internet standard that enables the transfer of email messages between servers, but does not send or compose the actual contents itself. Clients like Gmail or Outlook handle creation and storage of messages. SMTP only defines the protocol for routing the messages across networks until safely delivered to the intended recipient’s mailbox. Extensions to SMTP address common issues like security vulnerabilities for a safer email ecosystem overall.
Q: Can I use Postman or cURL to send SMTP email requests?
A: Yes, you can use API developer tools like Postman or cURL to directly send SMTP protocol requests to servers for testing. But manually formatting SMTP envelopes and message data can difficult.
Q: Does Microsoft Exchange use SMTP?
A: Yes. Exchange servers use SMTP to route and relay outgoing Internet-bound messages to the destination email servers. It also handles incoming SMTP mail flow from the Internet.
Q: Can SMTP send attachments?
A: No. SMTP mail only supports sending text, so attachments like documents and images can’t be transferred using SMTP alone. Mail clients handle encoding attachments into messages.
Q: Is SMTP insecure?
A: SMTP itself does not include encryption and security controls. But extensions like SMTPS and SMTP Auth allow encrypted connections and user authentication to improve security around SMTP transactions.
Q: Does SMTP use TCP or UDP?
A: SMTP communicates over TCP (“transmission control protocol”) which provides error-checking and reliability. Specifically it uses well-known TCP port 25 connections by default for non-encrypted SMTP.
Q: What is the difference between IMAP vs SMTP?
A: IMAP handles user mailbox access for retrieving messages. SMTP focuses only on transferring messages from clients. Together they allow users to send and receive emails.
Q: Can SMTP directly deliver mail to users?
A: No. SMTP servers can only transfer messages to other mail servers. They rely on separate mailbox servers like Exchange or systems offering POP/IMAP to handle user access and delivery of messages.
Q: Do webmail services use SMTP?
A: Yes. Webmail services like Gmail, Outlook.com and Yahoo Mail compose messages using their web interfaces, but in the backend use SMTP to send those emails out over the public internet.
Q: What are common SMTP response codes?
A: Common SMTP response codes are 250 (Request completed successfully) and 550 (Requested action not taken due to some error). 5xx errors tend to indicate issues with the request, while 2xx codes indicate success.
Q: How fast is SMTP email delivery?
A: On reliable connections, SMTP can deliver email almost instantly between trusted servers. But there are still often small delays for hopping between multiple systems, spam scanning, attachments etc. that may take seconds to rarely minutes for end-to-end public internet delivery.
Q: Can SMTP send to multiple recipients?
A: Yes. The SMTP RCPT TO command can be used multiple times to add many recipient email addresses for supporting bulk mailing to distribution lists. Rate limiting may still apply to curb large spam blasts.