You have to add the values to the MailBody string:
Public MailBody
Sub OddEvenNumber(iNumber)
If iNumber MOD 2 = 0 Or varBTWnr > "0" Then
MailBody = MailBody & IBAN : BE02 9793 3516 7640
Else
MailBody = MailBody & IBAN : BE11 9730 2045 8748
End If
End Sub