Email Security

    SMTP Gateways - What are these and Why are they Useful?

    SMTP is core to the delivery of corporate email, and SMTP gateways are core to corporate email security. This primer explains how they work.


    Key Points

    • Email use and the risks associated with using it are rising.
    • The Simple Mail Transport Protocol (SMTP) is an internet standard used to send, receive and relay email messages.
    • SMTP gateways perform actions on email, such as spam filtering, that can help protect organizations from email-borne attacks.

    As corporate dependence on email continues to grow, so do concerns about associated security risks. Indeed, Mimecast’s State of Email Security 2021 (SOES) report found 81% of IT and cybersecurity professionals noted an increase in the volume of email at their organizations during the past year. At the same time, more than two-thirds consider it likely (39%), extremely likely (26%) or inevitable (5%) that an email-borne attack will damage their businesses sometime this year, according to SOES.

    To understand the threat that email poses, it’s helpful to understand how email works in the first place. And there can’t be a discussion about email without a discussion about SMTP.

    What Is SMTP?

    SMTP stands for Simple Mail Transfer Protocol. Mail servers use this communications protocol to send, receive and relay mail messages over the Internet. SMTP typically works in conjunction with POP3 (Post Office Protocol) or IMAP (Internet Message Access Protocol), with SMTP used primarily for sending messages and POP3 or IMAP used primarily for receiving messages.

    Email clients, including Outlook, Gmail and Yahoo, use SMTP to send messages to an email server, where they’re processed and forwarded to recipients. Outgoing emails are usually sent using SMTP ports 587 or 465, while SMTP port 25 typically relays messages between email servers.

    What Is an SMTP Server?

    The primary purpose of an SMTP server is to send, receive and/or relay mail between email senders and receivers. The email client an organization uses sets one or more addresses for its SMTP server using the format When an email is sent, the SMTP server determines which server the message should be sent to and then sends the message. The intended recipient’s email system downloads the message and places it into the recipient’s email inbox. 

    During this process, the SMTP servers verify that outgoing email is from an active account. If an email cannot be delivered for some reason, the SMTP server sends the email back to the sender and informs them that, for example, the email address used was incorrect or the message was blocked from delivery by the receiving server.

    What Is SMTP Relay?

    SMTP relay is the process of one SMTP server communicating with another. SMTP relay is required when an email is sent between different domains, not when an email is sent to others with the same domain in their email addresses, such as users who work for the same organization. 

    SMTP relay services make it easier for organizations to send out a large volume of emails to many different domains, such is the case with an email marketing campaign, for example. SMTP relay services can send bulk and transactional emails, as well as optimize delivery rates –for example, helping to ensure an email is not marked as spam by a receiving server. 

    What Is an SMTP Gateway?
    Email phishing campaigns and other malicious attacks leveraging email rose significantly in 2020, according to the SOES report. Used as a strategic and layered approach to email security, an SMTP gateway helps protect sensitive information from becoming vulnerable to malware, spam and phishing attacks.

    With an SMTP gateway service, messages are routed from an organization to the gateway provider for outbound delivery, and mail exchange (MX) records are pointed to the provider for inbound delivery. The use of an SMTP gateway requires that SMTP send connectors for outbound mail and journaling be updated. MX records for inbound mail flow must also be updated.

    When selecting an SMTP gateway and gateway service provider, look for one that offers rich and actionable reporting based on an SMTP overview of the number and percentage of inbound, outbound and internal messages processed. 

    The Bottom Line

    Email use and its associated risk are growing. To ensure email security, organizations must develop their understanding of how email fundamentally works. SMTP is core to the delivery of corporate email, and SMTP gateways are core to protecting companies from the toll that spam, phishing and other email-borne attacks can take.  

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