I tried sending something for a postmark from the White House post office — unsuccessfully — and I'm hoping someone can figure out what's going on. Here is the envelope, front and back, with all the mailstream markings and stickers:


The contents were the following cover; a SASE; and a letter requesting a postmark:
1. The letter was returned with an "Insufficient Address, Unable to Forward" sticker. Now, "1700 Pennsylvania Ave NW, Washington, DC, 20006-4700" is most certainly a deliverable street address, in the sense that someone could drive there. It is
operational post office that's listed on usps.com.
Google Maps knows where it is. It seems unlikely to be that.
2. Could they not have a postmaster, and that's why it was undeliverable? Seems unlikely. They have retail hours, lobby hours, and PO Box service. I've never been there, but it sounds like a functioning post office that's open to the public to me.
3. Would it have been "sufficient" if I addressed it to 20006-9998 instead of 20006-4700? Doubtful. I've actually had my best success mailing to the post office street address, with a letter addressed to the postmaster, rather than flagging it with 9998.
4. Could it be a security issue? Perhaps they don't allow incoming mail because of proximity to the White House. Therefore, it would not be "insufficient address" so much as "we are unwilling to deliver this". That's where my money is.
5. Additional data: My letter was unopened and not tampered with; it took 16 calendar days from when it entered the mailstream until it was returned to me in California (which at least suggests that it made a cross-country trip); and it ended up with pen cancels and a cryptic "IA 690" scribbled on it.
6. OK, so what's "IA 690"? A code of some sort, presumably. I can infer that "IA" is "Insufficient Address", but the string "690" only shows up five times in the 1,250-page DMM, and none is relevant.
So, what's going on? Has anyone seen "IA 690" before? Is this indeed a security issue?
I'd love to hear answers/thoughts/theories.