What is disadvantage of SMTP?

Simple Mail Transfer Protocol (SMTP) is a standard protocol for sending emails across the Internet. It allows software to transmit emails reliably and efficiently.

While SMTP is essential for email services, it does come with some disadvantages:

Unencrypted connections

One of the biggest downsides of basic SMTP is that it sends messages in plain, unencrypted text. This means emails transmitted via SMTP can potentially be read by hackers through packet sniffing.

What is disadvantage of SMTP?

To resolve this, many email providers use SMTP extensions like SMTP over TLS (Transport Layer Security) to encrypt connections. However, the base SMTP protocol itself does not encrypt messages.

Susceptible to exploits

Like any protocol, SMTP has vulnerabilities that can be exploited by attackers:

  • Email injection: Hackers can inject malicious email headers to reroute messages or extract user data.
  • Spoofing: Fraudsters can spoof the sender address on outgoing emails. This facilitates phishing and spam campaigns.
  • Denial-of-service attacks: Spammers can flood SMTP servers with garbage traffic to overwhelm infrastructure and prevent legitimate emails from going through.

To prevent exploits, SMTP gateways need to be hardened with security filters and edge protection. Many providers also maintain IP reputation lists to block known spammers.

Not built for large attachments

A legacy protocol designed in the 1980s, SMTP was meant for sending small, text-based messages. It can run into issues when trying to handle large email attachments like high-res photos or videos.

Size limitations on attachments can cause messages to bounce. SMTP also provides limited transmission diagnostics, which makes troubleshooting attachment issues tedious.

No native chat/calendar options

Unlike modern platforms, SMTP itself has no native support for calendars, chats, contacts and other collaborative features now commonplace in email.

Enabling these capabilities requires bolt-on protocols like IMAP, POP3, ActiveSync and EWS. This added complexity reduces security and increases chances of sync errors across devices.

Metadata removed

As SMTP transfers emails between mail servers, it strips out much of the original metadata and formatting.

Links can get broken, images often won’t display by default, and file attachments can lose metadata like creation date or author.

This can negatively impact user experience and cause confusion if recipients don’t see emails as originally composed.

Key Takeaways

  • SMTP sends emails unencrypted, posing privacy risks
  • It’s vulnerable to exploits like spoofing and denial-of-service attacks
  • Size limitations result in large attachments getting blocked
  • No built-in support for calendars, chats, etc. Requires additional protocols
  • Strips out metadata leading to broken images/links in emails

Hopefully as SMTP gets updated or replaced over time, successors will resolve these disadvantages. For now, being aware of SMTP’s limitations allows admins to plan security measures and deliver better email reliability.


While crucial for Internet communication, SMTP comes from a simpler era of computing. Modern emails often desire complex capabilities that decades-old protocols never envisioned.

Until SMTP evolves or transitions fully to next-gen successors, its built-in lack of encryption, permissions, tracing and other features will continue posing challenges. But fortunately, understanding these core SMTP disadvantages allows IT teams to architect robust frameworks for security and delivery around it.

So rather than replacing SMTP wholesale, the path forward is to isolate and strengthen it – then innovate on top with modern protocols. This preserves compatibility while pushing email’s capabilities into the future.

Frequently Asked Questions About Disadvantages of SMTP

Is SMTP insecure?

Yes, the base SMTP protocol sends all data, including login credentials, as unencrypted plain text. This allows emails to be intercepted and read during transmission. Many providers now use SMTP TLS to encrypt connections.

Can SMTP be hacked?

Yes, SMTP has vulnerabilities hackers can exploit to access mail servers, reroute emails, extract user credentials, and launch spam/phishing campaigns. Hardening SMTP gateways and employing protective measures is necessary.

Does SMTP have spam filters?

No, SMTP itself does not include anti-spam or anti-virus filtering. Additional layers like SMTP proxies or email gateways usually handle filtering spam and malware before it reaches mailboxes.

Why was SMTP created?

SMTP was created in 1982 to bring a standard protocol for sending emails reliably over the early Internet, no matter the underling architecture. Before SMTP, email systems were siloed and unable to interoperate.

Does SMTP support read receipts?

No, basic SMTP does not have the capability to notify senders when an email has been opened or read. Modern email platforms can support read receipts using proprietary APIs and web protocols, but this lacks native support in SMTP.

Can SMTP send to multiple recipients?

Yes, a key benefit of SMTP is its native capability to easily address emails to multiple recipients without increased effort on the sender’s part. The receiving SMTP server replicates and sends messages to each addressed recipient.

Is SMTP inefficient?

SMTP can be inefficient for one-to-one email transferring because messages make extra network hops via intermediate SMTP mail relays rather than a direct client-to-server connection. But for one-to-many emails, SMTP vastly improves delivery efficiency.

Is SMTP outdated?

As an early Internet protocol, SMTP lacks many modern email features, security capabilities, and user comforts. But its reliable core SMTP architecture remains crucial for global email interoperability. Higher-level protocols have been added to supplement SMTP limitations.

Can SMTP queue emails offline?

Yes, most SMTP mail servers include message queueing capability so even when offline or encountering downtime, incoming emails remain safely stored and retries can continue attempting delivery. Once back online, queued messages will resume sending.

What port does SMTP use?

By default, SMTP communicates over TCP port 25. Alternatively, submissions of outgoing SMTP messages often use TCP port 587 instead, with or without TLS encryption on top. These standard ports allow easy email routing globally across the Internet.

Does Microsoft 365 use SMTP?

Yes, Microsoft 365 utilizes SMTP for sending and receiving emails in its hosted Office 365 email platform. Under the hood, protocols like Exchange Web Services (EWS) and ActiveSync proxy commands into SMTP operations to synchronize emails with Outlook mail clients.

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