|With Category-Manager you can group your Outlook categories, share them with other users, filter a folder by category, automatically categorize new emails, and more. You can use the Addin even for IMAP.
This macro is less about the receiver getting your message with an assigned category but more about you quickly finding the sent message in your own folder by using keywords. The script prompts you if a category is missing. You can then cancel the sending.
Private Sub Application_ItemSend(ByVal Item As Object, Cancel As Boolean)
On Error Resume Next
If Len(Item.Categories) = 0 Then
Msg = "Do you want to add a category?"
If MsgBox(Msg, vbYesNo Or vbQuestion) = vbYes Then
Cancel = True