Triggering of a mail on release of any PO for First time only

Hi Experts,

I have a requirement like:

The mail should be triggered to the approver only when the PO is released for the very first time using Tcode ME28 or ME29N after creation of PO using tcode ME21N.

For this I was using Badi ME_PURCHDOC_POSTED~Posted method. But I failed to recognize whether the PO is being release for the very first Time only or multiple times.

Please suggest your points.

Thanks in advance,

Sanjeev Kumar