We need to show Green tick icon in mailbox item as we are showing from VSTO Pluging using below code-
Now we need to set Green tick in mailbox item from web add-in So we tried custom properties as well as single extended value property to set on mailItem but still we are not able to show Green tick.
Please check raised git ticket as below-
Please give support to add custom icons to outlook mailbox items from web add-in. Please consider this feature request when we go through your planning process.