As outlined in the
<a href="https://dev.office.com/reference/add-ins/outlook/1.5/NotificationMessages" rel="nofollow noreferrer">NotificationMessages</a>
documentation, the
.ItemNotificationMessageType
property states...
In cases where the type is ProgressIndicator or ErrorMessage, an icon will be provided automatically and the message will not persist.
For this specific client (likely a Web browser or desktop Outlook), an "info" icon is displayed for notifications of type errorMessage
. If you wish to change the default icon, you can consider using the InformationalMessage
type, which gives you the ability to specify the icon
property. This property refers to an icon defined in the manifest under the Resources section, giving you flexibility in choosing any desired icon.