
The evolution of email: from the past to the present and future
March 13, 2024
What is SMTP?
SMTP is short for Simple Mail Transfer Protocol. It is a standard protocol for sending email over the Internet. It acts like a letter carrier, taking the email from the sender to the recipient's email server.
How SMTP works

How SMTP works
- The email sender writes the email and clicks "Send".
- The sender's email program connects to the email hosting provider's SMTP server.
- The sender's SMTP server verifies the recipient's email address.
- The sender's SMTP server delivers the email to the recipient's SMTP server.
- The recipient's SMTP server delivers the email to the recipient's mailbox.
Port used
- SMTP: port 25
- SMTPS (SMTP over SSL/TLS): port 465
- SMTP (submission): port 587
Limitations of SMTP
The Simple Mail Transfer Protocol (SMTP) is a fundamental protocol used for sending and relaying email messages across networks. While SMTP serves as the backbone for email communication, it also comes with certain limitations and considerations:
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- Message Size Limitations: SMTP imposes limits on the size of individual email messages that can be sent or received. This limit is typically defined by the email server's configuration and can vary depending on the server software and network policies. Exceeding message size limits can result in message rejection or delivery failures.
- Reliability Challenges: SMTP operates on a best-effort delivery model, which means it does not guarantee message delivery. Messages can be delayed, lost, or bounced due to various factors such as network issues, server outages, or recipient mailbox full errors.
- Security Concerns: SMTP was originally designed without built-in encryption mechanisms, making email transmissions susceptible to interception and eavesdropping. While modern implementations support encryption using protocols like STARTTLS or SMTPS, security remains a concern.
- Spam and Abuse: SMTP is vulnerable to abuse by spammers and malicious actors who exploit its open nature to send unsolicited or fraudulent emails. This has led to the development of anti-spam measures and sender authentication techniques like SPF, DKIM, and DMARC.
- Limited Metadata: SMTP headers provide limited metadata about the email, such as sender, recipient, subject, and timestamp. This can be insufficient for certain advanced use cases requiring richer message context or tracking capabilities.
- No Built-in Delivery Status Tracking: SMTP lacks inherent support for tracking the delivery status of emails beyond basic delivery notifications (DSNs). Monitoring the status of an email delivery, such as whether it was read by the recipient, requires additional mechanisms or services.
- Latency and Throughput: SMTP transactions involve multiple handshakes and negotiations between email servers, potentially leading to latency and reduced throughput, especially during peak periods or when processing large volumes of email traffic.
- Character Encoding Limitations: SMTP supports only ASCII text by default, which may pose challenges for sending emails containing non-ASCII characters or multilingual content. MIME extensions are commonly used to support encoding of non-textual data within email messages.
Despite these limitations, SMTP remains a widely adopted and essential protocol for email communication. Over time, enhancements and supplementary technologies have been developed to address many of these challenges and improve the overall reliability, security, and efficiency of email delivery.
Differences between SMTP and POP3/IMAP
- SMTP is used for sending emails.
- POP3/IMAP is used for receiving emails.
SMTP is an important protocol for sending emails on the Internet. Understanding how SMTP works will help you use email effectively.