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This documentation is curated by 2600Hz as part of the KAZOO open source project. Join our community forums here for peer support. Only features in the space are included as part of our 2600Hz Support Services plan.

Virtual Fax Machine (FaxBox)


The faxbox provides all the functionality of a virtual fax machine. A faxbox is a configurable entity in kazoo which can be used to send and receive faxes. For more information about how to configure a faxbox see the crossbar faxbox documentation.

Receiving Faxes#

The faxbox, like receive_fax can be attached to a call flow and used to receive faxes, though unlike receive_fax which requires a one to one relationship between the receive_fax call flow element and an email address, faxbox permits configuration of inbound recipient lists which will all receive a copy of the faxed document.

Email to Fax#

Kazoo fax application uses an SMTP server, fax_smtp to receive the outbound fax requests and initiate a fax transmission using the content attached to the email.

To initiate an email to fax, an email is sent by the user to the faxbox domain using the format: {destination_phone_number}@faxbox_domain.

Kazoo will determine which faxbox should be used for transmission based on the sender email address and destination domain. If the sender is authorized to send a fax via the faxbox, a fax will be transmitted to the destination_phone_number provided in the user-part of the to email address using the faxbox domain name specified in the domain-part of the to email address.

DNS and Email#

The Kazoo fax SMTP server is found by email servers by looking up the MX record of either the realm assigned to the account, a custom smtp address, or the generated faxbox domain name.

Custom SMTP Address#

The custom_smtp_email_address is a configurable parameter on the faxbox. This is used when the owner of the faxbox wants to register a unique domain name for the smtp server MX record instead of generating an address from defaults.


For simplicity, the accounts realm can also be used for white labeling the fax domain. This can be accomplished by registering a wildcard MX record for * When the realm is used, the account is identified.

Faxbox SMTP email address#

When a faxbox is created, a random faxbox smtp_email_address is generated using the default_smtp_domain defined in either the global system_config/fax document or at the account or parent accounts level, account config fax document. This address can be used directly if a wildcard MX record is registered for * Since the smtp_email_address is unique system wide for the faxbox, the faxbox is identified from the to header in the email and access to it validated by the from address, checking if it is associated with a user on the account's email address or the faxbox's smtp_permissions_list. The smtp domain address will be in the form XXXX.[default_smtp_domain].

The default_smtp_domain can be changed via sup using the following commands:

Customize the global default_smtp_domain: * sup kapps_config set fax default_smtp_domain your.domain.tld

Customize an accounts default_smtp_domain: * sup kapps_account_config [{ACCOUNT_ID}] set fax default_smtp_domain your.domain.tld

How Faxboxes Are Found For Email To Faxbox#

To determine the faxbox to use to send the fax, the fax_smtp server first looks at the domain part of the to email address to see if it is a faxbox smtp_email_address or custom_smtp_email_address. If a match is found, the from email address is validated against the associated the user of the smtp_permissions_list.

If no matches are found for the faxbox the domain part is checked against the account realm.

If no matches are found for the realm, smtp_email_address and custom_smtp_email_address, the email is rejected with a 554 not found SMTP error.

If the realm matches the domain part, the the from header in the email message is checked. If this is associated with a user on the account which is associated with a faxbox, the users fax box is selected. If no users are found associated with the faxbox, the senders email address is checked against the addresses configured in the faxboxes smtp_permissions_list and this faxbox is selected.

If no matches are found for the from email address in either user or smtp_permissions_list, the email is rejected with a 554 not allowed SMTP error.

Notes about DNS configuration for use in email to fax#

Generated smtp_email_address Addresses#

This is an example of a simple bind configuration for the faxboxes generated smtp_email_address addresses.

Note: this example assumes that is the default_smtp_domain configured in v2/system_config/fax.

    MX  0
        MX  0
        TXT "v=spf1 mx ptr -all"
mail-01     A   x.x.x.x
mail-02     A   x.x.x.x
*           MX  5
*           MX  5

Custom SMTP Addresses#

When a user customizes the SMTP address of the faxbox, the user must also add a DNS MX record to the domain. For example, if the user configured the SMTP address to be myfax.mydomain.tld, then the user would have to add the following DNS entry:

$ORIGIN mydomain.tld.
myfax       CNAME

If user has many faxboxes they can also add wildcards for the fax domain.

$ORIGIN fax.mydomain.tld.
*           CNAME
  • Important : add SPF policy to DNS to prevent email address spoofing

Google Cloud Printer#

Also faxes can be transmitted by linking your google developer account to the faxbox using a google cloud printer.


  • Google will render the document and deliver it as pdf to kazoo
  • The printer can be shared amongst users with google accounts without them being tied to kazoo
  • Can be used by any device with a browser connected to the internet
  • Native Drivers for Windows, OSX, Linux


To setup the google fax printer feature, you need a Google developer account. Go to the Google Developer console and create a new project or use an existing one. Go to Credentials and create a new Web Application Client ID or use an existing one. After creating the client id, note the following

  • CLIENT ID (OAuthId)
  • CLIENT SECRET (Secret)

You will need a reseller account or your master account id to configure this via kazoo.

Note: you can retrieve your master account with sup kapps_util get_master_account_id

Issue the following sup commands replacing with proper values:

  • sup kazoo_oauth_maintenance register_common_providers
  • sup kazoo_oauth_maintenance register_oauth_app {ACCOUNT_ID} {OAUTH_ID} {EMAIL} {SECRET} google
  • sup kapps_account_config set {ACCOUNT_ID} fax cloud_oauth_app {OAUTH_ID}
  • sup kapps_account_config set {ACCOUNT_ID} fax enable_cloud_connector true

Every new faxbox will register a new google cloud printer waiting to be claimed. The url for claiming the printer is in the document and is also returned on a faxbox creation. After it is claimed (a user with a google account goes into the url and claims it) the printer will be put online. the google user can share the printer with whomever they wish and all the jobs will be assigned to the account_id.