Secure Receipt policies allow inbound and outbound messages to be received and sent securely using Transport Layer Security (TLS) technology. TLS is designed to reduce the risk of eavesdropping, interception, and alteration of mail sent across the internet.
Consider the following before configuring a policy:
- When configuring route based TLS, the following policies are required to ensure the entire transmission is encrypted:
- Secure Receipt policy: This encrypts data between the sending mail server and us (i.e. how we receive a message).
- Secure Delivery policy: This encrypts data between Mimecast and the destination mail server (i.e. how we deliver a message).
- TLS technology protects confidentiality and data integrity by encrypting connections between servers, so that messages are transmitted through a secure tunnel. It uses SMTP over an SSL encrypted tunnel, and requires an installed third party certificate at each end of the tunnel. See the "Self Signed Certificates" section below for further information.
- We support connections using TLS 1.0, 1,1 and 1.2 for AES-256, RC4, MD5 and AnonDHE.
Self Signed Certificates
To configure and use TLS, each mail server involved in the sending and receipt of a message must have an SSL certificate from a public root certificate authority installed and configured. By default, TLS connections take place over port 25.
For Secure Receipt policies, we act as the server. The client application connects to us and checks the server certificate to see if it is acceptable. Typically it would be, and the connection is established. If a self signed certificate is used by the client, it wouldn't succeed in the connection attempt.
A configuration option can be enabled by Mimecast Support that allows the connection to be established. In this scenario we act as the client, and the customer application acts as the server. We will then verify the certificate.
Configuring a Secure Receipt Policy
To configure a Secure Receipt policy:
- Log on to the Administration Console.
- Click on the Administration menu item. A menu drop down is displayed.
- Click on the Gateway | Policies menu item. The Gateway Policy Editor is displayed.
- Click on Secure Receipt. A list of policies is displayed.
- Either select the:
- Policy to be changed.
- New Policy button to create a policy.
- Complete the Options section as required:
Option Description Policy Narrative Provide a description for the policy to allow you to easily identify it in the future. Select Option
Select one of the following values from the drop down list:
Delivery Option Description Default Uses Opportunistic TLS as described below. Enforced TLS Email is only delivered if the remote email server accepts TLS. If TLS is not configured, the connection will be dropped and the email delivery rejected.When configuring TLS on Mimecast using SMTP Enforced, ensure the recipients mail server is configured to accept TLS messages. Failure to do so means all email delivered using this policy will fail. It is recommended to test this communication before enforcing it across your entire organization. Opportunistic TLS TLS is always attempted first when sending a message, but if not accepted by the remote mail server, the message is delivered using standard SMTP.
- 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: Option Description 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 least specific. The options are: Option Description Everyone Includes all email users (i.e. internal and external). This option is only available in the "Emails From" section. Internal Address Includes only internal organization addresses. External Address Includes only external organization addresses. This option is only available in the "Emails From" section. Email Domain Enables you to specify a domain name to which this policy is applied. The domain name is entered in the Specifically field. Address Groups 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 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 Enables you to specify an SMTP address. The email address is entered in the Specifically field.
- 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 will automatically be 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 are selected, no date is required. Policy Override This overrides the default order that policies are applied. If there are multiple applicable policies, this policy is applied first unless more specific policies of the same type are configured with an override. 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.
- Click on the Save and Exit button.
To verify if TLS is being applied as expected once your policy is configured, send a test email and check the receipt / delivery view of the message.