You can create a macro using VBA to handle your request, something like the following example:
Option Explicit
Private Sub Application_ItemSend(ByVal Item As Object, Cancel As Boolean)
Dim olApp As Outlook.Application
Dim olNamespace As Outlook.NameSpace
Dim olAccounts As Outlook.Accounts
Dim olAccount As Outlook.Account
Set olApp = Outlook.Application
Set olNamespace = olApp.GetNamespace("MAPI")
Set olAccounts = olNamespace.Accounts
' Assuming 1st account in the list is the default account
Set olAccount = olAccounts.Item(1)
' Check if the From email address is different from the default account
If Item.SentOnBehalfOfName <> olAccount.SmtpAddress Then
Dim response As VbMsgBoxResult
response = MsgBox("Are you sure you want to send this email from a non-default account?", vbYesNo + vbExclamation + vbDefaultButton2, "Warning")
If response = vbNo Then
Cancel = True
End If
End If
' Release objects
Set olAccount = Nothing
Set olAccounts = Nothing
Set olNamespace = Nothing
Set olApp = Nothing
End Sub