MailTips are notifications within Outlook, Outlook on the Web, and mobile versions of Outlook, that indicate an email you are sending meets some special criteria.

General MailTips: (original list sourced here)

• Invalid Internal Recipient – appears if the sender attempts to send an email to a non-existent internal contact (identified by the internal domain).
• Restricted Recipient – applies if the recipient is restricted via Message Delivery Restrictions. Blocking certain recipients on the mail flow level will not bring up the MailTip.
• External Recipients – MailTip shows up for messages sent to external domains. It is not possible to make it appear for chosen domains only, it is an on/off toggle kind of setting.
• Large Audience – shows up whenever a message is going to be sent to many recipients. By default, the limit is set to 25 users or more. Exchange counts the number of users in distribution lists and groups.
• Moderated Recipient – if the recipients’ group is subject to message approval, a MailTip will inform the sender about this fact.
• Reply-All on Bcc – hitting the Reply-all button while being a BCC Recipient will result in a warning.
• Oversize Message – this MailTip warns that the message about to be sent cannot be delivered due to its size. The settings that are checked are organization-wide settings and recipient-specific settings. Limits defined on Exchange send and receive connectors will not result in displaying this MailTip.

MailTips relying on mailbox data

The following two types of MailTips are different because they rely on mailbox data, and not on organization-wide settings.

• Out of office – this MailTip type is probably one of the most often to be seen. The notification shows up when the recipient has automatic replies turned on. It informs that an automatic reply will be sent back and shows its contents.
• Mailbox full – shows up whenever the message cannot be delivered because the recipient’s Mailbox is full.

How to configure your MailTips in Outlook

1. Launch Outlook
2. Click File, click Options, Click Mail and scroll until you find the MailTips section
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3. Click MailTips Options...
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   2. Here you may select which MailTips appear or if they appear at all.