Policies - Configuring Greylisting Policies

This article contains information on greylisting, a compliance check to reduce spam by temporarily rejecting emails from unknown senders, and provides steps to configure or bypass greylisting policies in Mimecast.

Greylisting is a default compliance check applied to all inbound messages from connections not previously seen by us. Provided the sender's mail server (Message Transfer Agent - MTA) complies with best practice guidelines (RFC compliance), the message is successfully delivered.

The vast majority of spam is sent from applications designed specifically for that purpose. These adopt the "fire-and-forget" method, where they attempt to send spam to one or more MX hosts for a domain, but never attempt a retry. By using greylisting policies, any messages sent from an incorrectly configured MTA aren't accepted, helping to reduce the amount of spam.

The Greylisting Process

Greylisting looks at the following pieces of information for the delivery attempt:

• IP address of the MTA
• Envelope sender address
• Envelope recipient address

With this information, we have a unique relationship for that particular SMTP session:

• If we've never seen this information before, a server busy status (451 Resource is Temporarily Unavailable) is issued. This is a temporary failure and is maintained for 60 seconds, forcing the sending server to queue and retry.
• A correctly configured MTA always attempts to retry the message's delivery. If the MTA retries after 60 seconds and before the 12-hour upper limit, the message is accepted.
• If the message is not retried in this 12-hour period, an entry is logged in the Rejection Viewer as "Sender Failed to Retry" (12 hours after the initial attempt). See Rejected and Deferred Messages.
• If the sending MTA attempts again after 12 hours from the initial attempt, the greylisting process restarts.

By default, an Exchange server retries to send a message every 10 minutes until the message retries expire.

Usage Considerations

Consider the following before creating a policy:

• All email connections that have been subjected to greylisting are logged-in connection attempts.
• Any sender email address, domain, or IP address added to the Auto Allow or Permitted Senders list isn't subjected to greylisting.
• A greylisting policy is created by default by Mimecast Support during the Implementation process, configured to apply to all inbound traffic. There may be instances where you experience difficulty receiving emails from legitimate senders whose MTA hasn't been correctly configured. If the sender's MTA doesn't comply with RFC standards, but their messages are deemed safe for your organization, you can create a greylisting bypass policy.

Configuring a Greylisting Policy

To configure a Greylisting policy:

1. Log in to the Mimecast Administration Console.
2. Navigate to Gateway | Policies.
3. Select Greylisting.
4. Either select the:

• • Policy to be changed.
  • New Policy button to create a policy.

5. Complete the Options section as required:

Option	Description
Policy Narrative	Enter a policy description that allows you to identify it.
Select Option	Select whether to apply greylisting or to take no action (bypass).

6. Complete the Emails From and Emails To sections as required:

Field / Option	Description
Addresses Based On	Specify the email address characteristics the policy is based on. This option is only available in the "Emails From" section: The Return Address: This default setting applies the policy to the SMTP address match, based on the message's envelope or true address (i.e., the address used during SMTP transmission).
Applies From / To	Specify the Sender characteristics the policy is based on. For multiple policies, you should apply them from the most to the least specific. The options are: Everyone: Includes all internal and external users. This option is only available in the "Emails From" section. Internal Address: Includes only internal addresses. External Address: Includes only external addresses. This option is only available in the "Emails From" section. Email Domain: This enables you to specify a domain to which this policy is applied. The domain name is entered in the Specifically field. Address Groups: This Enables you to specify a directory or local group. If this option is selected, click on the Lookup button to select a group from the Profile Group field. Once a group has been selected, you can click on the Show Location field to display the group's path. Address Attributes: This enables you to specify a predefined attribute. The attribute is selected from the Where Attribute drop-down list. Once the attribute is specified, an attribute value must be entered in the Is Equal To field. This can only be used if attributes have been configured for user accounts. Individual Email Address: This enables you to specify an SMTP address. The email address is entered in the Specifically field.

7. Complete the Validity section as required:

Field / Option	Description
Enable / Disable	Use this to enable (default) or disable a policy. If a date range has been specified, the policy is automatically disabled when the end of the configured date range is reached.
Set Policy as Perpetual	If the policy's date range has no end date, this field displays "Always On" meaning that the policy never expires.
Date Range	Use this field to specify a start and/or end date for the policy. If the Eternal option is selected, no date is required.
Policy Override	Applies Auto Allow to the recipient's email address for all internal end users, avoiding spam checks when users receive mail from this recipient.
Bi-directional	If selected, the policy is applied when the policy's recipient is the sender, and the sender is the recipient.
Source IP Ranges (n.n.n.n/x)	Enter any required Source IP Ranges for the policy. These only apply if the source IP address used to transmit the message data falls inside or matches the range(s) configured. IP ranges should be entered in CIDR notation.

8. Select the Save and Exit button.