Sending emails with hidden "to" box

Hi Hatters,
Lately i've noticed I received a few emails (into my gmail inbox) where the "To" box is empty. Meaning the sender somehow managed to hide the address he used.

I looked every where on the web for email services who can do that, checked every setting on my mail provider and tried endless searches with no results.
This has got nothing to do with "BCC" copies if this is what you are about to answer. If someone BCCed me I will see my address under "BCC" in gmail, however here there is no BCC field, only an empty "To" line.

Now of course that if I go and read the full email header I can see which address was used, but most people do not know how to do it.
I would really want to be able to do this for my email marketing.

Attached are two screenshots of two emails from my email client (gmail) and the full emails with header and footer (without mail client intervention). Sorry the later one is attached as photo but since it has emails and links i cannot paste it as text on the forum

Get them all the time, asked my host what this is about and they couldn't tell me, so maybe someone here know about how this is possible.

I found out that this is related to the way your mail client uses BCC feature.
For example with many versions of apple mail, if you put someone in the BCC field of your email, they will get the empty 'to'box and will not show BCC.
This is because the source mail client does something wrong when writing the envelope of the email and the dest cannot parse it correctly.

I also found some bulk mail softwares that can send emails like this such as maxbulk mailer.

with PHP you can define the senders address...
check PHP mail function.