Having a super strange issue with just one of our vendors in Japan. In Mid-May,
our email started bouncing back to them. The bounce back says (real info has been
redacted as per company policy):

The original message was received at Thu, 10 Jul 2014 15:32:15 -0500
from ip-xxx-xxx-xxx-xxx.sub.isp.com [xxx.xxx.xxx.xxx]

----- The following addresses had permanent fatal errors -----
(reason: 553 Invalid address syntax)

----- Transcript of session follows -----
... while talking to amxi.aics.ne.jp.:
>>> DATA
<<< 553 Invalid address syntax
550 5.1.1 <first.last@vendorname.co.jp>... User unknown
<<< 503 No recipients specified

The addresses we sent the email to for this vendor are valid, as we can send to them using Gmail or Yahoo! and the messages get through to the recipients. And before mid-May, our messages were being accepted by them to these very same addresses.
Our mail relay provider has tested the mail server via telnet and e-mail to same exact addresses is accepted.
Our mail relay provider and I have verified the MX records look fine for their domain.
We can send to any other companies in Japan successfully.

Not getting much help / ideas at this point. Their network admin has done some checking, but claims he has no detailed info in his logs (Exchange, I believe) to say why their system thinks the address is either invalid or empty (no recipients specified). I have asked they check for a spare character in front of the address, or an empty character, etc. but have not gotten any help there.
I have asked if they have made any recent changes, again not much of an answer there.

Yesterday, we were assigned a new purchasing contact there, an address we had not sent mail to before. The first message went thru and we were hopeful things were maybe fixed. Today sending mail to that new message bounced back with the same
bounceback errors.

I am looking for any ideas here. This is an issue with the one vendor, it does not affect any other company, vendor, or person to whom we send e-mail via Groupwise, just these guys. Has anyone ever experienced this before with Groupwise? Is it something in Groupwise, or something else? We've not applied any new patches to Groupwise since earlier this year, well before the issue
started in May. Our mail relay provider did switch their screening product in mid-April and made the cutover the end of April,
and we had no issues then sending to anyone. It's just a real head-scratcher for everyone.