Understanding MailTips

Applies to: Exchange Server 2010

MailTips are informative messages displayed to users while they're composing a message. Microsoft Exchange Server 2010 analyzes the message, including the list of recipients to which it's addressed, and if it detects a potential problem, it notifies the user with MailTips prior to sending the message. With the help of the information provided by MailTips, senders can adjust the message they're composing to avoid undesirable situations or non-delivery reports (NDRs).

The following unproductive messaging scenarios are common in any messaging environment:

  • NDRs resulting from messages that violate restrictions configured in an organization such as message size restrictions or maximum number of recipients per message.
  • NDRs resulting from messages sent to recipients that don't exist, recipients that are restricted, or users whose mailboxes are full.
  • Sending messages to users with automatic replies configured.

All of these scenarios involve the user sending a message, expecting it to be delivered, and instead receiving a response stating that the message isn't delivered. Even in the best-case scenario, like the automatic reply, these events result in lost productivity. In the case of an NDR, this scenario could result in a costly call to the Help desk.

There are also several scenarios where sending a message won't result in an error, but can have undesirable, even embarrassing consequences:

  • Messages sent to extremely large distribution groups.
  • Messages sent to inappropriate distribution groups.
  • Messages inadvertently sent to recipients outside your organization.
  • Selecting Reply to All to a message that was received as a Bcc recipient.

All of these problematic scenarios can be mitigated by informing users of the possible outcome of sending the message as they're composing the message. For example, if senders know that the size of the message they're trying to send exceeds the corporate policy, they won't attempt to send the message. Similarly, if senders are notified that the message they're sending will be delivered to people outside the organization, they are more likely to ensure that the content and the tone of the message are appropriate.

By addressing the scenarios listed earlier, MailTips can help you to:

  • Reduce the cost of processing and storing messages by preventing NDRs.
  • Reduce the volume of Help desk calls caused by NDRs.
  • Increase productivity by avoiding communications that won't succeed, for example, breaking the cycle of sending an e-mail message, receiving an automatic reply, and then redirecting the message.
  • Inform your users as they compose e-mail messages about various policies configured in your organization that impose limits on the messages sent.
  • Direct your users to the correct distribution groups.
  • Reduce the risk of inadvertent disclosure of information to people outside your organization.


MailTips aren't designed to enforce specific policies. They simply inform the senders about the nature of the message they're composing so they can make necessary adjustments.

Looking for management tasks related to MailTips? See Managing MailTips.

MailTips in Exchange 2010

The following table provides a list of MailTips in Exchange 2010.

Exchange 2010 MailTips

MailTip Scenario

Invalid Internal Recipient

The Invalid Internal Recipient MailTip is displayed if the sender adds a recipient that appears to be internal to the organization but doesn't exist in Active Directory.

This could happen if the sender addresses a message to a user who is no longer with the company but whose address resolves due to name resolution cache or an entry in the sender's Contacts folder. It can also happen if the sender types an SMTP address with a domain for which Exchange is authoritative and the address doesn't resolve to an existing recipient.

The MailTip indicates the invalid recipient and gives the sender the option to remove the recipient from the message.

Mailbox Full

The Mailbox Full MailTip is displayed if the sender adds a recipient whose mailbox is full and your organization has implemented a Prohibit Receive restriction for mailboxes over a specified size.

The MailTip indicates the recipient whose mailbox is full and gives the sender the option to remove the recipient from the message.

The MailTip is accurate at the time of display. If the message isn't immediately sent, the MailTip will be updated every two hours. This also applies to messages that were saved in the Drafts folder and reopened after two hours.

Automatic Replies

The Automatic Replies MailTip is displayed if the sender adds a recipient who has turned on automatic replies.

The MailTip indicates the recipient has automatic replies turned on and also displays the first 250 characters of the automatic reply configured by the recipient.

The MailTip is accurate at the time of display. If the message isn't immediately sent, the MailTip will be updated every two hours. This also applies to messages that were saved in the Drafts folder and reopened after two hours.


A custom MailTip is displayed if the sender adds a recipient for whom a customized MailTip is configured.

A custom MailTip can be useful for providing specific information about a recipient. For example, you can create a custom MailTip for a distribution group explaining its purpose to reduce its misuse.

By default, custom MailTips aren't displayed if the sender isn't allowed to send to that recipient. In that case, the Restricted Recipient MailTip is displayed. However, you can change this configuration and have the custom MailTip also display. For more information about configuring custom MailTips, see Configure Custom MailTips for Recipients.

Restricted Recipient

The Restricted Recipient MailTip is displayed if the sender adds a recipient for which delivery restrictions are configured prohibiting this sender from sending messages.

The MailTip indicates the recipient to which the sender isn't allowed to send messages and gives the sender the option to remove the recipient from the message. It also clearly informs the sender that the message won't be delivered if sent.

If the restricted recipient is an external recipient, or if it's a distribution group that contains external recipients, this information is also provided to the sender. However, the following MailTips, if applicable, are suppressed:

  • Automatic Replies
  • Mailbox Full
  • Custom MailTip
  • Moderated Recipient
  • Oversize Message

External Recipients

The External Recipients MailTip is displayed if the sender adds a recipient that's external, or adds a distribution group that contains external recipients.

This MailTip informs senders if a message they're composing will leave the organization, helping them make the correct decisions about wording, tone, and content.

By default, this MailTip is turned off. You can turn it on using the Set-TransportConfig cmdlet. For details, see Configure Organizational Settings for MailTips.

Large Audience

The Large Audience MailTip is displayed if the sender adds a distribution group that has more than the large audience size configured in your organization. By default, Exchange displays this MailTip for messages to distribution groups that have more than 25 members. For information about configuring the large audience size for your organization, see Configure Organizational Settings for MailTips.

The size of distribution groups isn't calculated each time. Instead, the distribution group information is read from the Group Metrics data. For more information about Group Metrics, see Understanding Group Metrics.

Moderated Recipient

The Moderated Recipient MailTip is displayed if the sender adds a recipient that's moderated.

The MailTip indicates which recipient is moderated and informs the sender that this may result in delay of the delivery.

If the sender is also the moderator, this MailTip isn't displayed. It's also not displayed if the sender has been explicitly allowed to send messages to the recipient (by adding the sender's name to the Accept Messages Only From list for the recipient).

Reply-All on Bcc

The Reply-All on Bcc MailTip is displayed if the sender receives a Bcc copy of a message and selects Reply to All.

When a user selects Reply to All to such a message, the fact that the user received a Bcc of that message is revealed to the rest of the audience to which the message was sent. In almost all cases, this is an undesirable situation, and this MailTip informs the user of this condition.

Oversize Message

The Oversize Message MailTip is displayed if the message the sender is composing is larger than configured message size limits in your organization.

The MailTip is displayed if the message size violates one of the following size restrictions:

  • Maximum send size setting on the sender's mailbox
  • Maximum receive size setting on the recipient's mailbox
  • Maximum message size restriction for the organization
  • Maximum Request Length setting (for Microsoft Office Outlook Web App only)
Due to the complexity of the implementation, the message size limits on the connectors in your organization aren't taken into account.

MailTips Architecture

MailTips are implemented as a Web service in Exchange 2010. When a sender is composing a message, the client software makes an Exchange Web service call to the server running Exchange 2010 with the Client Access server role installed to get the list of MailTips. The Exchange 2010 server responds with the list of MailTips that apply to that message, and the client software displays the MailTips to the sender.

The following messaging clients support MailTips:

  • Outlook Web App
  • Microsoft Outlook 2010

The following actions by the sender trigger MailTips to be evaluated or updated:

  • Add a recipient
  • Add an attachment
  • Reply or Reply to all
  • Open a message, which is already addressed to recipients, from the Drafts folder

The client caches the MailTips so removing and adding the same recipient won't result in the client querying the Client Access server again.

When the Client Access server is queried, it compiles the list of applicable MailTips and returns them all in one batch. This way, all MailTips will be displayed to the user at the same time, and not arrive one at a time, distracting the sender.

The Client Access server uses the following sources to compile MailTips for a specific message:

  • Active Directory
  • Recipient mailboxes
  • Local Group Metrics data

Each time a sender adds a recipient to a message, a sequence of events occurs to evaluate MailTips, as shown in the following figure.

MailTips evaluation
MailTips evaluation workflow

As shown in the preceding figure:

  1. The mail client queries the Web service on the Client Access server for MailTips that apply to the recipients in the message.
  2. The Client Access server gathers MailTip data:
    1. The Client Access server queries Active Directory and reads Group Metrics data.
    2. If the recipient is a mailbox that's located on a Mailbox server in the local site, the Client Access server queries the Mailbox server to gather the Automatic Replies and Mailbox Full MailTips. If the recipient's mailbox is in another site, the Client Access server requests MailTips information from the Client Access server in the remote site.
    3. The Client Access server in the remote site queries the local Mailbox server for MailTip data.
    4. The remote Client Access server proxies the results back to the requesting Client Access server.
  3. The Client Access server returns MailTip data back to the client.

Limits on MailTips

MailTips are subject to the following restrictions:

  • MailTips aren't supported when working in offline mode with Outlook.
  • When a message is addressed to a distribution group, the MailTips for individual recipients that are members of that distribution group aren't evaluated. However, if any of the members is an external recipient, the External Recipients MailTip is displayed, which shows the sender the number of external recipients in the distribution group.
  • If the message is addressed to more than 200 recipients, individual mailbox MailTips aren't evaluated due to performance reasons.
  • Custom MailTips are limited to 250 characters.
  • If the sender starts composing a message and leaves it open for an extended period of time, the Automatic Replies and Mailbox Full MailTips will be evaluated every two hours.

MailTips in Complex Topologies

MailTips that rely on the recipients' mailbox data are evaluated by making a connection to the Mailbox servers that hold those recipients. The Mailbox Full and Automatic Replies MailTips are in this category. For these MailTips, the Client Access server directly queries the Mailbox server using RPC, but only if the recipients are in the same site. For recipients that reside in other sites or forests, this information is gathered via server-to-server Web requests between Client Access servers.

Performance and Scalability

The following table lists some of the common performance concerns regarding MailTips and how these concerns are addressed.

Common performance concerns for MailTips

Performance concerns Mitigation

Discovering the size of distribution groups and dynamic distribution groups, and whether they include external recipients, seems like an expensive operation.

Distribution groups and dynamic distribution groups aren't evaluated when a message is being composed. This information is calculated daily by the Group Metrics generation service and is distributed to all Client Access servers. For more information, see Understanding Group Metrics.

Discovering the delivery restrictions for all recipients in a message with a large number of recipients can be resource- intensive.

Delivery restrictions aren't evaluated if a message has more than 200 recipients. Also, delivery restrictions aren't evaluated for the members of distribution groups and dynamic distribution groups. This is done only for recipients that are explicitly added to the message.

If a user expands a distribution group, delivery restrictions for all members will be evaluated, as long as the total number of recipients doesn't exceed 200.

It may take a long time to collect information from various sites in complex topologies.

If the requested information isn't returned within 10 seconds, the operation will time out. Exchange won't display any new MailTips to the sender after 10 seconds.