Office 365 Outlook application will display "External" in the message heading when your YCCD email account receives a message from a email address outside the YCCD email address. This "External" message is available on the Web and mobile version of Outlook.
Please review sending name after the "External" message before opening the email or selecting any links within the email.
Below are sample images of the "External" message.
Mobile Device - IPhone
Mobile Device - Android
Identification of external sender displayed on your mobile device
Please review other Security Knowledge Base Articles listed in the Additional Resources below.
You can find additional Security Articles at this link, Information Security
Email Security
Email Encryption
Online Security