Use mail flow rules to route email based on a list of words, phrases, or patterns in Exchange Online

In Exchange Online organizations or standalone Exchange Online Protection (EOP) organizations without Exchange Online mailboxes, you can use mail flow rules (also known as transport rules) to find and act on messages that contains specific words. This action can help your users comply with your organization's email policies.

For a short list of words or phrases, you can use the Exchange admin center (EAC). For a longer list, you can use Exchange Online PowerShell or standalone EOP PowerShell to import the words from a text file.

If your Exchange Online organization uses Data loss prevention (DLP), see Data loss prevention for additional options for identifying and routing email that contains sensitive information (DLP is not available in standalone EOP).

Example 1: Use a short list of unacceptable words

If your list of words or phrases is short, you can create a rule using the Exchange admin center. For example, if you want to make sure no one sends email with bad words or with misspellings of your company name, internal acronyms or product names, you could create a rule to block the message and tell the sender. Note that words, phrases, and patterns are not case sensitive.

This example blocks messages with common typos.

Rule showing blocking a message based on text patterns.

Example 2: Use a long list of unacceptable words

If your list of words, phrases, or patterns is long, you can put them in a text file with each word, phrase, or pattern on its own line. Use Exchange Online PowerShell to read in the list of keywords into a variable, create a mail flow rule, and assign the variable with the keywords to the mail flow rule condition. For example, the following script takes a list of misspellings from a file called C:\My Documents\misspelled_companyname.txt.

$Keywords=Get-Content "C:\My Documents\misspelled_companyname.txt"
New-TransportRule -Name "Block messages with unacceptable words" -SubjectOrBodyContainsWords $Keywords -SentToScope "NotInOrganization" -RejectMessageReasonText "Do not use internal acronyms, product names, or misspellings in external communications."

Using phrases and patterns in the text file

The text file can contain regular expressions for patterns. These expressions are not case-sensitive. Common regular expressions include:

Expression Matches
. Any single character
* Any additional characters
\d Any decimal digit
[character_group] Any single character in character_group.

For example, this text file contains common misspellings of Microsoft.

[mn]icro soft

To learn how to specify patterns using regular expressions, see Regular Expression Reference.