It doesn't smell right.  Some question you may ask yourself...

• The IRS will not outsource mail processing to a third country, AFAIK.
• What is the notice number?  Does the content conform to the standard format?
• Does it have the taxpayer's SSN truncated?  If so, are the digits correct?
• IRS generally uses addresses with dedicated ZIP codes and not street addresses for PDS.  And addresses for PDS don't come with Mail Stop numbers.

There is probably a phone number they suggest the taxpayer to call for questions.  It is most likely not an IRS number if you check (although it may look like one with a digit changed or numbers transposed).  They are probably hoping the taxpayer, knowing he/she is overseas, would call rather than correspond by mail so that they could extract information from him/her during the call.

I'd call the PPS to check that out, provided you are a third party designee or have a PoA.  Even if it's real, I wouldn't submit anything without first verifying the authenticity of the notice with the IRS.

If the notice turns out to be a scam but they have your client's mailing address, know that they filed an international return, and the SSN (whether or not truncated) is correct, your client should check his/her credit report, etc., and take the necessary actions as his/her personal data may have been breached.

@The Real Halloween Here is the PPS contact info-I would call them
https://www.irs.gov/tax-professionals/practitioner-priority-service-r

Many thanks for all your valuable comments.

If the IRS does not outsource its mail to Estonia, this makes the letter a real fake.  

FYI, the contact numbers that the letter suggests TP should call for questions are the IRS numbers (800-829-1040 and +1-267-941-1000 for outside the US).  However, the SSN is not truncated at all.  The letter has no notice # but letter #.  Also, the mailing address of the requested information is Mail Stop 6126 AUSC, which is different from publication 1546 (3651 S. Interregional Highway 35
Stop 1005 AUSC Austin, TX 78741) as suggested by @qbteachmt.

As I do not have PoA Form 2848, I would suggest TP to make a verification call to the IRS (+1-267-941-1000 the IRS International Hotline).  If the IRS letter is legit, we will respond.  If not, we will ignore.  Please comment if you have on this.  Thanks.

One thing bothers me....what good would it do for a scammer to have the response sent to an authentic IRS address?  The only question is the Mail Stop number and that sends it to a specific department at that address. I have a bit of deja vu in that that Mail Stop number seems familiar to me. That said, if my client had received a letter with so many people questioning it, I would ask further questions, too.

@The Real Halloween  Does it look like any of these google images for fake letters? https://tinyurl.com/wddxgmm You could do your own search based on the letter number "fake irs letter ### austin tx"
Did you e-file? or paper file?
Are you a third party designee?
Have you check the IRS "Where's my refund?" https://www.irs.gov/refunds
https://www.google.com/search?q=irs+letter+with+estonia+stamps could be of interest

@The Real Halloween Those are genuine IRS numbers.  It's looking like the IRS may be using mail drop overseas to dispatch notices to foreign addresses.  But I'd still call the IRS to double check.

Please let us know what you find out.

Thanks for all your very valuable comments.

@George4Tacks Thanks for your https://tinyurl.com/wddxgmm  The letter that TP received is similar to the Google image on the IRS scam letter circulating in Adair County, MO by KTVO Saturday, February 27th 2016.  TP filed e-file through PTO and i am not the third party designee.  I am checking "Where's my refund? and will get back to you all once TP contacts the IRS for verification.

Personally, I wouldn't use Check My Refund to access a client's account without PoA.

TP called the IRS International Hotline at 267-941-1000 and was told by the IRS agent that the letter is from the IRS.  Therefore, the IRS did send letters from Estonia, untruncated SSN,  different mail stop address from p1546 and asking copies of return that it already had.  Thanks again for all your time and comments!

Good to know.  Thanks for the update!

"... Item being requested such as page 2 of 2018 Form 1040 is not in existence."

Actually the official F 1040 does has page one & two (on that infamous postcard).

"We" browbeat Intuit into at least allowing the option of printing it all on just one page.