Do Until Function returning Sub or Function not defined error

Try the other syntax of Do until:

Do Until condition
action
Loop

Private Sub CommandButton1_Click()
'
'set starting point under header

row_number = 1

Do Until IsEmpty(Cells(row_number, "A"))
    row_number = row_number + 1
    Call SendEmail(Sheet1.Range("A" & row_number), Sheet1.Range("B" & row_number), Sheet1.Range("C2"))
Loop 
 
End Sub

This will first check for instances matching the condition and then execute the action. It should prevent the macro from trying to send an e-mail to the blank cell it stops on