Introduction

MailScanner is one of the most widely used email security gateways for Linux mail servers. It scans incoming and outgoing emails for viruses, spam, phishing attempts, and potentially dangerous attachments.

By default, MailScanner may block certain file extensions based on its security policies. In some environments, organizations need to allow additional document formats such as .docx or custom extensions like .dox for business communication.

This guide explains how to configure MailScanner to allow these file extensions safely.

Why MailScanner Blocks Attachments

MailScanner uses filename rules to determine whether an attachment should be:

  • Allowed
  • Denied
  • Renamed
  • Quarantined

Blocking potentially dangerous file types helps prevent:

  • Malware infections
  • Ransomware
  • Macro viruses
  • Executable attachments disguised as documents

Sometimes legitimate document types must be permitted for internal users.

Step 1: Locate the File Type Rules

MailScanner stores filename rules under:

/etc/MailScanner/

Depending on your distribution, the file may be:

filename.rules.conf

or

filename.rules

Find the active configuration:

grep -i "Filename Rules" /etc/MailScanner/MailScanner.conf

Example output:

Filename Rules = %etc-dir%/filename.rules.conf

Step 2: Open the Rules File

sudo vi /etc/MailScanner/filename.rules.conf

or

sudo nano /etc/MailScanner/filename.rules.conf

Step 3: Check Existing Rules

You may see entries like:

deny     \.exe$
deny     \.bat$
deny     \.scr$
allow    \.pdf$
allow    \.doc$
allow    \.xls$

Step 4: Allow DOCX Files

If .docx is blocked or missing, add:

allow    \.docx$

Example:

allow    \.doc$
allow    \.docx$
allow    \.xlsx$
allow    \.pptx$

Step 5: Allow Custom DOX Files

If your organization uses a custom extension such as .dox, add:

allow    \.dox$

Example:

allow    \.doc$
allow    \.docx$
allow    \.dox$

Example Configuration

# Microsoft Office

allow    \.doc$
allow    \.docx$
allow    \.xls$
allow    \.xlsx$
allow    \.ppt$
allow    \.pptx$

# Custom Documents

allow    \.dox$

# PDF

allow    \.pdf$

Step 6: Save the File

After making the changes:

:wq

or simply save if using Nano.

Step 7: Restart MailScanner

Reload the service:

sudo systemctl restart MailScanner

Verify the service status:

sudo systemctl status MailScanner

Expected output:

Active: active (running)

Step 8: Test the Configuration

Send a test email with:

  • test.docx
  • sample.dox

Verify that:

  • Email is delivered
  • Attachment is not renamed
  • Attachment is not quarantined
  • No MailScanner warnings appear

Check MailScanner Logs

Monitor logs while testing:

tail -f /var/log/maillog

or

tail -f /var/log/mail.log

You should see entries indicating the message was processed successfully.

If Attachments Are Still Blocked

Check whether another MailScanner rule is overriding your configuration.

Search for .docx:

grep -R "docx" /etc/MailScanner/

Search for .dox:

grep -R "dox" /etc/MailScanner/

Also review:

Filetype Rules
Dangerous Content Scanning
Archive Rules
Incoming Work Directory

Verify MailScanner Configuration

Run:

MailScanner --lint

A successful configuration returns:

Infected message scanner: OK

SpamAssassin: OK

MailScanner.conf: OK

Resolve any reported errors before restarting the service.

Conclusion

MailScanner provides flexible attachment filtering that helps protect email infrastructure while allowing legitimate business documents. By updating the filename rules to include .docx and custom extensions like .dox, administrators can support organizational workflows without compromising overall email security.

Always validate configuration changes using MailScanner --lint, monitor your mail logs, and allow only trusted file types to maintain a secure and reliable email environment.

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