What is email security software?

Email security software protects organizations against spam, phishing, malware, and business email compromise by inspecting messages, enforcing SPF/DKIM/DMARC authentication, sandboxing suspicious attachments, and quarantining threats. It also covers archiving, encryption, and continuity for compliance.

When does building email security make sense?

Building makes sense for organizations with data-residency requirements that prohibit routing email through cloud filtering services. Rspamd and Proxmox Mail Gateway are documented production alternatives for teams with Linux operations capacity.

When does buying email security make sense?

Buying is the default because Microsoft Defender for Office 365 is bundled in E3/E5 licenses many enterprises already hold. Separate purchases make sense for AI-based BEC detection or compliance archiving beyond what the platform provides.

What are the main email security vendors?

Representative vendors include Proofpoint, Mimecast, Microsoft Defender for Office 365, Abnormal Security. B4 Pro scores the full set.

What is business email compromise (BEC) and why is it hard to filter?

BEC is a category of email fraud where attackers impersonate executives or trusted partners using legitimate-looking domains and plausible message content, without malware or suspicious links. Traditional rule-based filtering misses it because there's nothing technically wrong with the message — which is why behavioral AI trained on normal executive communication patterns is the primary detection approach.

Security & Compliance · Engineering, IT & AI

Should you build or buy Email Security?

Email security software protects organizations against spam, phishing, malware, and business email compromise by inspecting inbound and outbound messages, enforcing authentication standards like SPF, DKIM, and DMARC, sandboxing suspicious attachments, and quarantining threats before they reach users. It also covers email archiving, encryption, and continuity for compliance and operational resilience.

The build-vs-buy decision for email security turns largely on whether your organization is already paying for a productivity suite that bundles filtering natively, and whether the compliance, archiving, and continuity requirements on top of basic filtering justify a separate specialized purchase; the specifics decide it.

Domain: Security & Compliance
Function: Engineering, IT & AI
Industries: Cross-industry

Last assessed June 2026 · re-scored quarterly via The Continuum.

Build it, buy it, or bridge?

	Build it	Buy it	Bridge (buy, then extend)
Cost shape	Rspamd/Proxmox Mail Gateway free; Amavis/SpamAssassin stack well-documented; labor for ongoing rule updates and compliance overhead	Proofpoint/Mimecast separate license on top of M365; Defender for Office 365 bundled in E3/E5	Platform bundled filtering plus specialized add-on for advanced sandboxing or archiving
Time to value	Days to deploy Rspamd; weeks to tune; compliance coverage requires ongoing configuration	Microsoft Defender active in hours for M365 environments; full configuration in days	Platform filtering active immediately; specialized add-on configured alongside
Differentiation captured	None; invisible utility infrastructure	AI-based BEC detection from global threat intelligence; compliance archiving and legal hold	Platform for baseline; specialized vendor for BEC or archiving requirements
AI feasibility today	Rspamd, SpamAssassin, Proxmox Mail Gateway documented production alternatives; gap is AI-based BEC detection trained on global traffic	Abnormal Security's behavioral AI trained on global enterprise email patterns; Microsoft threat intel at platform scale	Platform AI for common threats; specialized vendor for behavioral BEC detection
Who it fits	Privacy-first orgs with Linux ops capacity and data-residency constraints on email routing	Most enterprises, especially those already in M365 where Defender is effectively included	M365 orgs adding behavioral BEC detection or specialized archiving on top of platform filtering

The B4 call

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When building Email Security makes sense

Building email security infrastructure makes the most sense for organizations with strict data-residency requirements where routing email through a cloud-delivered filtering service is unacceptable, and teams with existing Linux operations capacity. Rspamd is a production-grade open-source filtering engine in documented deployments replacing commercial alternatives, covering SPF/DKIM/DMARC/ARC enforcement, Lua-scriptable detection rules, and spam scoring. Proxmox Mail Gateway provides a full open-source gateway alternative for organizations running on-premises mail infrastructure. For data-sensitive environments that can't send message content to a third-party platform, self-hosted filtering is a real and maintained option. The constraint is compliance: legal hold, archiving, continuity, and on-premises delivery scenarios require years of specialized product development that commercial vendors have already done — and building equivalent capabilities would consume engineering resources disproportionate to the value.

When buying Email Security makes sense

Buying email security is the default for most organizations because the bundling math has largely decided it. Microsoft Defender for Office 365 ships inside E3 and E5 licenses that most enterprises already hold. Google Workspace includes comparable filtering for Gmail environments. Paying separately for Proofpoint or Mimecast means buying a second filtering layer on top of platform protection already running at partial capacity. The cases where a separate purchase makes sense are behavioral business email compromise detection — where Abnormal Security's models trained on global enterprise email patterns catch sophisticated attacks that rules-based filtering misses — and comprehensive archiving and legal hold, where compliance depth is the product rather than an afterthought. If your email security requirement is standard spam and phishing filtering, what's already bundled in your productivity suite is very likely sufficient.

Email security has largely been decided by bundling. Microsoft Defender for Office 365 ships inside the E3 and E5 licenses that most enterprises already hold, and Google Workspace includes comparable filtering for Gmail environments. For the majority of orgs, paying separately for Proofpoint or Mimecast means buying a second layer on top of platform protection they're already using at partial capacity.

The build case has a narrow window: teams with strict data residency requirements and existing Linux ops capacity can run Rspamd or a Proxmox Mail Gateway stack in production, and it's documented and mature enough to replace commercial filtering for inbound spam and phishing. Where buying stays compelling is compliance: legal hold, archiving, continuity, and on-prem delivery scenarios that commercial vendors have spent years building out. If your email security requirement is mostly 'filter spam and block phishing,' the commodity tier of the market, including what's already bundled in your productivity suite, is very likely sufficient.

Representative vendors

ProofpointMicrosoft Defender for Office 365 and 3 more, scored in B4 Pro

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Frequently asked

What is email security software?: Email security software protects organizations against spam, phishing, malware, and business email compromise by inspecting messages, enforcing SPF/DKIM/DMARC authentication, sandboxing suspicious attachments, and quarantining threats. It also covers archiving, encryption, and continuity for compliance.
When does building email security make sense?: Building makes sense for organizations with data-residency requirements that prohibit routing email through cloud filtering services. Rspamd and Proxmox Mail Gateway are documented production alternatives for teams with Linux operations capacity.
When does buying email security make sense?: Buying is the default because Microsoft Defender for Office 365 is bundled in E3/E5 licenses many enterprises already hold. Separate purchases make sense for AI-based BEC detection or compliance archiving beyond what the platform provides.
What are the main email security vendors?: Representative vendors include Proofpoint, Mimecast, Microsoft Defender for Office 365, Abnormal Security. B4 Pro scores the full set.
What is business email compromise (BEC) and why is it hard to filter?: BEC is a category of email fraud where attackers impersonate executives or trusted partners using legitimate-looking domains and plausible message content, without malware or suspicious links. Traditional rule-based filtering misses it because there's nothing technically wrong with the message — which is why behavioral AI trained on normal executive communication patterns is the primary detection approach.

The B4 Index scores every software category on two axes, strategic differentiation and AI feasibility, to classify it Build, Buy, Bridge, or Beware. See the full methodology.

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