AegisAI Perspectives · 083
The best outbound email DLP for Microsoft 365 checks who gets the data and whether the sender usually sends it there, then runs in audit before blocking.
No single product is best: the best outbound email DLP for Microsoft 365 checks who receives the data, not only what it contains. Misdelivery made up 72% of action varieties involving end users in breaches1. AegisAI checks what an outbound email holds, who it is going to and whether that is normal for the sender, then applies your policy17.
The criteria below apply to any product you trial, including the DLP built into Microsoft 365. False positives get their own deep dive in whether outbound DLP will drown your team in false positives. This page covers the whole choice: the built-in layer, misdirected recipients, context, testing and admin access.
Microsoft Purview DLP, the built-in Microsoft 365 feature, is the baseline any added layer has to beat on your own mail. In Exchange, new email messages are scanned and an alert is generated when a policy matches14. A policy can block a message and, through a policy tip, let the user override and record a justification13.
Detection rests on sensitive information types. Each type is defined by a pattern with a primary element and supporting elements, plus a confidence level and proximity6. The primary element is a regular expression, a keyword list, a keyword dictionary or a function7.
Ask the vendor: which checks does your product add to the Purview policies we already run, and which does it repeat?
Misdelivery is the error that outbound DLP exists to catch. In the Verizon 2025 DBIR, the top three Miscellaneous Errors were Misdelivery, Misconfiguration and Publishing error3. That pattern held 1,476 incidents, 1,449 with confirmed data disclosure2. In those error breaches, the data was Personal in 95% of cases4.
A pattern rule sees a nine-digit number, not an autocomplete slip to the wrong Jordan. The check has to compare the recipient with the sender's history: first contact, personal domain, or a lookalike of the usual partner.
Ask the vendor: show me a payroll file sent to a new outside address with a name close to the usual recipient. What fired, and why?
NIST defines data loss prevention as content inspection plus contextual analysis of transaction attributes, the recipient among them5. A built-in type raises its confidence level when it finds more supporting evidence near the match8. That evidence sits inside the message, not in the sender's habits.
The cost of pattern-only rules is tuning debt. The outbound DLP page on aegisai.com puts it plainly: regex rules fire on routine payroll and miss the leak to a personal inbox, so teams tune them down until real leaks pass24.
Ask the vendor: the same file goes to the payroll provider every month. Does your product treat it differently when it goes to a personal Gmail address?
A DLP policy should run on live mail without acting before it enforces anything. In Microsoft Purview simulation mode, a policy runs as if it were enforced, without any actual enforcement9. Simulations can run for up to 15 days10, and tuning there reduces false positives without impact on users11.
One limit matters for email. For Exchange, only items that are new during the simulation are evaluated12, so the test needs real outbound traffic for its full run. Ask any added layer for the same audit step on live mail.
Ask the vendor: can a policy run in audit on live outbound mail, and how do I raise it to alert or block?
The user prompt is part of the control. Microsoft recommends policy tips to raise awareness before a policy moves from simulation mode to more restrictive modes15. An override with a recorded justification turns a false positive into data for tuning.
Ask the vendor: which actions run before delivery on Microsoft 365, which run on Google Workspace, and where is each justification logged?
A DLP alert copies sensitive data into a second place. Ask whether the flagged snippet is masked in the console, who can open it, and whether each open is logged. Ask the same of Purview sensitivity labels: can the product read them on outbound mail, and does it need extra rights for encrypted messages?
Ask the vendor: show me an alert on a social security number, as an analyst without admin rights sees it.
List the Purview DLP policies you run and what each one blocks.
Send a sensitive file to a new outside address that resembles a usual recipient.
Send the same file to its usual recipient and compare the verdicts.
Forward a company file to a personal address in the sender's own name.
Run every policy in audit on live outbound mail for the full trial.
Count alerts per 1,000 outbound messages for pattern rules and for the context check.
Confirm which actions run before delivery on each platform you use.
Check that flagged snippets are masked until an admin opens them.
Record every user override and its justification.
| Criterion | API-based | Gateway | Built-in platform controls | Managed service |
|---|---|---|---|---|
| Wrong recipient | Can compare the recipient with the sender's history; ask for a test | Depends on the product; ask for a test | Patterns plus supporting evidence in the message | Depends on the analyst queue |
| Context over patterns | Can weigh the sender and recipient together with the content | Depends on the rules set at the relay | Confidence rises with more supporting evidence | Rules set by the provider |
| Testing before enforcement | Ask for an audit mode on live mail | Ask whether rules can log without acting | Simulation for up to 15 days | Ask how a new rule is trialled |
| User prompt at send | Depends on the platform's actions | Usually a bounce or a hold after send | Policy tip with override and justification | Depends on the tooling |
| Mail flow change | None for detection when it reads through the API | MX or connector change | None | Depends on the provider's tools |
Illustrative example, figures are fictional. Lakemont Credit Union (fictional) has 450 staff on Microsoft 365 and runs Purview DLP with two policies. In a three week trial, the team runs an added layer in audit on all outbound mail. Purview alerts on 212 messages, and 198 of them are the monthly payroll file going to the payroll provider. The context check alerts on 9 messages. One is the payroll file sent to a personal address in the sender's own name, and one is a member list sent to a lookalike of the auditor's domain. The team raises those two rules to alert, and keeps Purview for its labels.
AegisAI reads who is sending sensitive data, to whom, and whether that is normal, then applies your policy16. Early access runs on the connection you already have. Start a policy in audit, then decide. Actions depend on the platform, and pre-delivery blocking is not available on Google Workspace. During early access we confirm which actions your tenant has.
Start a policy in audit on your own outbound mail, then decide, or read the mechanism on the outbound email DLP page. For the inbound side of the same tenant, see how to choose email security for Microsoft 365.
The one that checks the recipient and the sender's history as well as the content, and runs in audit on live mail before it blocks. Test it against Purview DLP on the same outbound traffic.
It scans new Exchange messages against sensitive information types and can block with an override. Its patterns weigh evidence inside the message, so test whether misdirected mail to a usual-looking recipient is caught.
In the Verizon 2025 DBIR, Misdelivery made up 72% of action varieties involving end users. It was the top Miscellaneous Error once again.
Run it on live outbound mail until the alert reasons match your analysts' reading. Purview simulation runs for up to 15 days.
Some products read them. AegisAI can read Purview sensitivity labels on outbound mail and use them to decide what its agents analyze.
